diff --git a/testing/run_tests.py b/testing/run_tests.py
index 5107ec49d18e2..966a02778e34d 100755
--- a/testing/run_tests.py
+++ b/testing/run_tests.py
@@ -908,6 +908,7 @@ def build_dart_host_test_list(build_dir):
           ],
       ),
       (os.path.join('flutter', 'testing', 'litetest'), []),
+      (os.path.join('flutter', 'testing', 'skia_gold_client'), []),
       (
           os.path.join('flutter', 'tools', 'api_check'),
           [os.path.join(BUILDROOT_DIR, 'flutter')],
diff --git a/testing/skia_gold_client/lib/skia_gold_client.dart b/testing/skia_gold_client/lib/skia_gold_client.dart
index 5f654a4bc37c8..5dde4d8608003 100644
--- a/testing/skia_gold_client/lib/skia_gold_client.dart
+++ b/testing/skia_gold_client/lib/skia_gold_client.dart
@@ -3,10 +3,11 @@
 // found in the LICENSE file.
 
 import 'dart:convert';
-import 'dart:io';
+import 'dart:io' as io;
 
 import 'package:crypto/crypto.dart';
 import 'package:engine_repo_tools/engine_repo_tools.dart';
+import 'package:meta/meta.dart';
 import 'package:path/path.dart' as path;
 import 'package:process/process.dart';
 
@@ -19,25 +20,61 @@ const String _instance = 'flutter-engine';
 
 /// Whether the Skia Gold client is available and can be used in this
 /// environment.
-bool get isSkiaGoldClientAvailable => Platform.environment.containsKey(_kGoldctlKey);
+bool get isSkiaGoldClientAvailable => SkiaGoldClient.isAvailable();
 
 /// Returns true if the current environment is a LUCI builder.
-bool get isLuciEnv => Platform.environment.containsKey(_kLuciEnvName);
-
-/// Whether the current task is run during a presubmit check.
-bool get _isPresubmit => isLuciEnv && isSkiaGoldClientAvailable && Platform.environment.containsKey(_kPresubmitEnvName);
-
-/// Whether the current task is run during a postsubmit check.
-bool get _isPostsubmit => isLuciEnv && isSkiaGoldClientAvailable && !Platform.environment.containsKey(_kPresubmitEnvName);
+bool get isLuciEnv => io.Platform.environment.containsKey(_kLuciEnvName);
 
 /// A client for uploading image tests and making baseline requests to the
 /// Flutter Gold Dashboard.
-class SkiaGoldClient {
+interface class SkiaGoldClient {
   /// Creates a [SkiaGoldClient] with the given [workDirectory].
   ///
   /// [dimensions] allows to add attributes about the environment
   /// used to generate the screenshots.
-  SkiaGoldClient(this.workDirectory, { this.dimensions, this.verbose = false});
+  SkiaGoldClient(
+    this.workDirectory, {
+    this.dimensions,
+    this.verbose = false,
+    io.HttpClient? httpClient,
+    ProcessManager? processManager,
+    StringSink? stderr,
+    Map<String, String>? environment,
+  }) : httpClient = httpClient ?? io.HttpClient(),
+       process = processManager ?? const LocalProcessManager(),
+       _stderr = stderr ?? io.stderr,
+        _environment = environment ?? io.Platform.environment;
+
+  /// Whether the client is available and can be used in this environment.
+  static bool isAvailable({
+    Map<String, String>? environment,
+  }) {
+    final String? result = (environment ?? io.Platform.environment)[_kGoldctlKey];
+    return result != null && result.isNotEmpty;
+  }
+
+  /// Returns true if the current environment is a LUCI builder.
+  static bool isLuciEnv({
+    Map<String, String>? environment,
+  }) {
+    return (environment ?? io.Platform.environment).containsKey(_kLuciEnvName);
+  }
+
+  /// Whether the current environment is a presubmit job.
+  bool get _isPresubmit {
+    return
+        isLuciEnv(environment: _environment) &&
+        isAvailable(environment: _environment) &&
+        _environment.containsKey(_kPresubmitEnvName);
+  }
+
+  /// Whether the current environment is a postsubmit job.
+  bool get _isPostsubmit {
+    return
+        isLuciEnv(environment: _environment) &&
+        isAvailable(environment: _environment) &&
+        !_environment.containsKey(_kPresubmitEnvName);
+  }
 
   /// Whether to print verbose output from goldctl.
   ///
@@ -45,18 +82,24 @@ class SkiaGoldClient {
   /// ordinarily be set to true.
   final bool verbose;
 
+  /// Environment variables for the currently running process.
+  final Map<String, String> _environment;
+
+  /// Where output is written for diagnostics.
+  final StringSink _stderr;
+
   /// Allows to add attributes about the environment used to generate the screenshots.
   final Map<String, String>? dimensions;
 
   /// A controller for launching sub-processes.
-  final ProcessManager process = const LocalProcessManager();
+  final ProcessManager process;
 
   /// A client for making Http requests to the Flutter Gold dashboard.
-  final HttpClient httpClient = HttpClient();
+  final io.HttpClient httpClient;
 
   /// The local [Directory] for the current test context. In this directory, the
   /// client will create image and JSON files for the `goldctl` tool to use.
-  final Directory workDirectory;
+  final io.Directory workDirectory;
 
   String get _tempPath => path.join(workDirectory.path, 'temp');
   String get _keysPath => path.join(workDirectory.path, 'keys.json');
@@ -72,9 +115,9 @@ class SkiaGoldClient {
   /// Indicates whether the client has already been authorized to communicate
   /// with the Skia Gold backend.
   bool get _isAuthorized {
-    final File authFile = File(path.join(_tempPath, 'auth_opt.json'));
+    final io.File authFile = io.File(path.join(_tempPath, 'auth_opt.json'));
 
-    if(authFile.existsSync()) {
+    if (authFile.existsSync()) {
       final String contents = authFile.readAsStringSync();
       final Map<String, dynamic> decoded = json.decode(contents) as Map<String, dynamic>;
       return !(decoded['GSUtil'] as bool);
@@ -85,10 +128,14 @@ class SkiaGoldClient {
   /// The path to the local [Directory] where the `goldctl` tool is hosted.
   String get _goldctl {
     assert(
-      isSkiaGoldClientAvailable,
+      isAvailable(environment: _environment),
       'Trying to use `goldctl` in an environment where it is not available',
     );
-    return Platform.environment[_kGoldctlKey]!;
+    final String? result = _environment[_kGoldctlKey];
+    if (result == null || result.isEmpty) {
+      throw StateError('The environment variable $_kGoldctlKey is not set.');
+    }
+    return result;
   }
 
   /// Prepares the local work space for golden file testing and calls the
@@ -107,7 +154,7 @@ class SkiaGoldClient {
       '--luci',
     ];
 
-    final ProcessResult result = await _runCommand(authCommand);
+    final io.ProcessResult result = await _runCommand(authCommand);
 
     if (result.exitCode != 0) {
       final StringBuffer buf = StringBuffer()
@@ -120,13 +167,12 @@ class SkiaGoldClient {
         ..writeln('stderr: ${result.stderr}');
       throw Exception(buf.toString());
     } else if (verbose) {
-      print('stdout:\n${result.stdout}');
-      print('stderr:\n${result.stderr}');
+      _stderr.writeln('stdout:\n${result.stdout}');
+      _stderr.writeln('stderr:\n${result.stderr}');
     }
   }
 
-  Future<ProcessResult> _runCommand(List<String> command) {
-    print(command.join(' '));
+  Future<io.ProcessResult> _runCommand(List<String> command) {
     return process.run(command);
   }
 
@@ -135,8 +181,8 @@ class SkiaGoldClient {
   /// The `imgtest` command collects and uploads test results to the Skia Gold
   /// backend, the `init` argument initializes the current test.
   Future<void> _imgtestInit() async {
-    final File keys = File(_keysPath);
-    final File failures = File(_failuresPath);
+    final io.File keys = io.File(_keysPath);
+    final io.File failures = io.File(_failuresPath);
 
     await keys.writeAsString(_getKeysJSON());
     await failures.create();
@@ -163,7 +209,7 @@ class SkiaGoldClient {
       throw Exception(buf.toString());
     }
 
-    final ProcessResult result = await _runCommand(imgtestInitCommand);
+    final io.ProcessResult result = await _runCommand(imgtestInitCommand);
 
     if (result.exitCode != 0) {
       final StringBuffer buf = StringBuffer()
@@ -176,8 +222,8 @@ class SkiaGoldClient {
         ..writeln('stderr: ${result.stderr}');
       throw Exception(buf.toString());
     } else if (verbose) {
-      print('stdout:\n${result.stdout}');
-      print('stderr:\n${result.stderr}');
+      _stderr.writeln('stdout:\n${result.stdout}');
+      _stderr.writeln('stderr:\n${result.stderr}');
     }
 
   }
@@ -209,13 +255,12 @@ class SkiaGoldClient {
   /// allowed to be different.
   Future<void> addImg(
     String testName,
-    File goldenFile, {
+    io.File goldenFile, {
     double differentPixelsRate = 0.01,
     int pixelColorDelta = 0,
     required int screenshotSize,
   }) async {
     assert(_isPresubmit || _isPostsubmit);
-
     if (_isPresubmit) {
       await _tryjobAdd(testName, goldenFile, screenshotSize, pixelColorDelta, differentPixelsRate);
     }
@@ -235,7 +280,7 @@ class SkiaGoldClient {
   /// comparison being evaluated.
   Future<void> _imgtestAdd(
     String testName,
-    File goldenFile,
+    io.File goldenFile,
     int screenshotSize,
     int pixelDeltaThreshold,
     double maxDifferentPixelsRate,
@@ -244,20 +289,25 @@ class SkiaGoldClient {
 
     final List<String> imgtestCommand = <String>[
       _goldctl,
-      'imgtest', 'add',
-      if (verbose) '--verbose',
-      '--work-dir', _tempPath,
-      '--test-name', cleanTestName(testName),
-      '--png-file', goldenFile.path,
+      'imgtest',
+      'add',
+      if (verbose)
+      '--verbose',
+      '--work-dir',
+      _tempPath,
+      '--test-name',
+      _cleanTestName(testName),
+      '--png-file',
+      goldenFile.path,
       // Otherwise post submit will not fail.
       '--passfail',
       ..._getMatchingArguments(testName, screenshotSize, pixelDeltaThreshold, maxDifferentPixelsRate),
     ];
 
-    final ProcessResult result = await _runCommand(imgtestCommand);
+    final io.ProcessResult result = await _runCommand(imgtestCommand);
 
     if (result.exitCode != 0) {
-final StringBuffer buf = StringBuffer()
+      final StringBuffer buf = StringBuffer()
         ..writeln('Skia Gold received an unapproved image in post-submit ')
         ..writeln('testing. Golden file images in flutter/engine are triaged ')
         ..writeln('in pre-submit during code review for the given PR.')
@@ -272,8 +322,8 @@ final StringBuffer buf = StringBuffer()
         ..writeln('stderr: ${result.stderr}');
       throw Exception(buf.toString());
     } else if (verbose) {
-      print('stdout:\n${result.stdout}');
-      print('stderr:\n${result.stderr}');
+      _stderr.writeln('stdout:\n${result.stdout}');
+      _stderr.writeln('stderr:\n${result.stderr}');
     }
   }
 
@@ -282,8 +332,8 @@ final StringBuffer buf = StringBuffer()
   /// The `imgtest` command collects and uploads test results to the Skia Gold
   /// backend, the `init` argument initializes the current tryjob.
   Future<void> _tryjobInit() async {
-    final File keys = File(_keysPath);
-    final File failures = File(_failuresPath);
+    final io.File keys = io.File(_keysPath);
+    final io.File failures = io.File(_failuresPath);
 
     await keys.writeAsString(_getKeysJSON());
     await failures.create();
@@ -301,7 +351,7 @@ final StringBuffer buf = StringBuffer()
       '--passfail',
       '--crs', 'github',
       '--patchset_id', commitHash,
-      ...getCIArguments(),
+      ..._getCIArguments(),
     ];
 
     if (tryjobInitCommand.contains(null)) {
@@ -313,7 +363,7 @@ final StringBuffer buf = StringBuffer()
       throw Exception(buf.toString());
     }
 
-    final ProcessResult result = await _runCommand(tryjobInitCommand);
+    final io.ProcessResult result = await _runCommand(tryjobInitCommand);
 
     if (result.exitCode != 0) {
       final StringBuffer buf = StringBuffer()
@@ -326,8 +376,8 @@ final StringBuffer buf = StringBuffer()
         ..writeln('stderr: ${result.stderr}');
       throw Exception(buf.toString());
     } else if (verbose) {
-      print('stdout:\n${result.stdout}');
-      print('stderr:\n${result.stderr}');
+      _stderr.writeln('stdout:\n${result.stdout}');
+      _stderr.writeln('stderr:\n${result.stderr}');
     }
   }
 
@@ -342,7 +392,7 @@ final StringBuffer buf = StringBuffer()
   /// comparison being evaluated.
   Future<void> _tryjobAdd(
     String testName,
-    File goldenFile,
+    io.File goldenFile,
     int screenshotSize,
     int pixelDeltaThreshold,
     double differentPixelsRate,
@@ -351,15 +401,19 @@ final StringBuffer buf = StringBuffer()
 
     final List<String> tryjobCommand = <String>[
       _goldctl,
-      'imgtest', 'add',
+      'imgtest',
+      'add',
       if (verbose) '--verbose',
-      '--work-dir', _tempPath,
-      '--test-name', cleanTestName(testName),
-      '--png-file', goldenFile.path,
+      '--work-dir',
+      _tempPath,
+      '--test-name',
+      _cleanTestName(testName),
+      '--png-file',
+      goldenFile.path,
       ..._getMatchingArguments(testName, screenshotSize, pixelDeltaThreshold, differentPixelsRate),
     ];
 
-    final ProcessResult result = await _runCommand(tryjobCommand);
+    final io.ProcessResult result = await _runCommand(tryjobCommand);
 
     final String resultStdout = result.stdout.toString();
     if (result.exitCode != 0 &&
@@ -375,8 +429,8 @@ final StringBuffer buf = StringBuffer()
         ..writeln();
       throw Exception(buf.toString());
     } else if (verbose) {
-      print('stdout:\n${result.stdout}');
-      print('stderr:\n${result.stderr}');
+      _stderr.writeln('stdout:\n${result.stdout}');
+      _stderr.writeln('stderr:\n${result.stderr}');
     }
   }
 
@@ -409,14 +463,14 @@ final StringBuffer buf = StringBuffer()
   Future<String?> getExpectationForTest(String testName) async {
     late String? expectation;
     final String traceID = getTraceID(testName);
-    await HttpOverrides.runWithHttpOverrides<Future<void>>(() async {
+    await io.HttpOverrides.runWithHttpOverrides<Future<void>>(() async {
       final Uri requestForExpectations = Uri.parse(
         '$_skiaGoldHost/json/v2/latestpositivedigest/$traceID'
       );
       late String rawResponse;
       try {
-        final HttpClientRequest request = await httpClient.getUrl(requestForExpectations);
-        final HttpClientResponse response = await request.close();
+        final io.HttpClientRequest request = await httpClient.getUrl(requestForExpectations);
+        final io.HttpClientResponse response = await request.close();
         rawResponse = await utf8.decodeStream(response);
         final dynamic jsonResponse = json.decode(rawResponse);
         if (jsonResponse is! Map<String, dynamic>) {
@@ -424,7 +478,7 @@ final StringBuffer buf = StringBuffer()
         }
         expectation = jsonResponse['digest'] as String?;
       } on FormatException catch (error) {
-        print(
+        _stderr.writeln(
           'Formatting error detected requesting expectations from Flutter Gold.\n'
           'error: $error\n'
           'url: $requestForExpectations\n'
@@ -438,30 +492,10 @@ final StringBuffer buf = StringBuffer()
     return expectation;
   }
 
-  /// Returns a list of bytes representing the golden image retrieved from the
-  /// Skia Gold dashboard.
-  ///
-  /// The provided image hash represents an expectation from Skia Gold.
-  Future<List<int>>getImageBytes(String imageHash) async {
-    final List<int> imageBytes = <int>[];
-    await HttpOverrides.runWithHttpOverrides<Future<void>>(() async {
-      final Uri requestForImage = Uri.parse(
-        '$_skiaGoldHost/img/images/$imageHash.png',
-      );
-
-      final HttpClientRequest request = await httpClient.getUrl(requestForImage);
-      final HttpClientResponse response = await request.close();
-      await response.forEach((List<int> bytes) => imageBytes.addAll(bytes));
-    },
-      SkiaGoldHttpOverrides(),
-    );
-    return imageBytes;
-  }
-
   /// Returns the current commit hash of the engine repository.
   Future<String> _getCurrentCommit() async {
     final String engineCheckout = Engine.findWithin().flutterDir.path;
-    final ProcessResult revParse = await process.run(
+    final io.ProcessResult revParse = await process.run(
       <String>['git', 'rev-parse', 'HEAD'],
       workingDirectory: engineCheckout,
     );
@@ -479,7 +513,7 @@ final StringBuffer buf = StringBuffer()
   Map<String, dynamic> _getKeys() {
     final Map<String, dynamic> initialKeys = <String, dynamic>{
       'CI': 'luci',
-      'Platform': Platform.operatingSystem,
+      'Platform': io.Platform.operatingSystem,
     };
     if (dimensions != null) {
       initialKeys.addAll(dimensions!);
@@ -494,15 +528,15 @@ final StringBuffer buf = StringBuffer()
 
   /// Removes the file extension from the [fileName] to represent the test name
   /// properly.
-  String cleanTestName(String fileName) {
-    return fileName.split(path.extension(fileName))[0];
+  static String _cleanTestName(String fileName) {
+    return path.basenameWithoutExtension(fileName);
   }
 
   /// Returns a list of arguments for initializing a tryjob based on the testing
   /// environment.
-  List<String> getCIArguments() {
-    final String jobId = Platform.environment['LOGDOG_STREAM_PREFIX']!.split('/').last;
-    final List<String> refs = Platform.environment['GOLD_TRYJOB']!.split('/');
+  List<String> _getCIArguments() {
+    final String jobId = _environment['LOGDOG_STREAM_PREFIX']!.split('/').last;
+    final List<String> refs = _environment['GOLD_TRYJOB']!.split('/');
     final String pullRequest = refs[refs.length - 2];
 
     return <String>[
@@ -515,6 +549,7 @@ final StringBuffer buf = StringBuffer()
   /// Returns a trace id based on the current testing environment to lookup
   /// the latest positive digest on Skia Gold with a hex-encoded md5 hash of
   /// the image keys.
+  @visibleForTesting
   String getTraceID(String testName) {
     final Map<String, dynamic> keys = <String, dynamic>{
       ..._getKeys(),
@@ -528,4 +563,4 @@ final StringBuffer buf = StringBuffer()
 }
 
 /// Used to make HttpRequests during testing.
-class SkiaGoldHttpOverrides extends HttpOverrides { }
+class SkiaGoldHttpOverrides extends io.HttpOverrides { }
diff --git a/testing/skia_gold_client/pubspec.yaml b/testing/skia_gold_client/pubspec.yaml
index a728eb7ba69b8..a7ba15de53abb 100644
--- a/testing/skia_gold_client/pubspec.yaml
+++ b/testing/skia_gold_client/pubspec.yaml
@@ -16,19 +16,30 @@ environment:
 # relative to this directory into //third_party/dart, or //third_party/pkg
 dependencies:
   crypto: any
+  meta: any
   path: any
   engine_repo_tools: any
   process: any
 
+dev_dependencies:
+  litetest: any
+  process_fakes: any
+
 dependency_overrides:
+  async_helper:
+    path: ../../../third_party/dart/pkg/async_helper
   collection:
     path: ../../../third_party/dart/third_party/pkg/collection
   crypto:
     path: ../../../third_party/dart/third_party/pkg/crypto
   engine_repo_tools:
     path: ../../tools/pkg/engine_repo_tools
+  expect:
+    path: ../../../third_party/dart/pkg/expect
   file:
     path: ../../../third_party/dart/third_party/pkg/file/packages/file
+  litetest:
+    path: ../../testing/litetest
   meta:
     path: ../../../third_party/dart/pkg/meta
   path:
@@ -37,5 +48,9 @@ dependency_overrides:
     path: ../../third_party/pkg/platform
   process:
     path: ../../third_party/pkg/process
+  process_fakes:
+    path: ../../tools/pkg/process_fakes
+  smith:
+    path: ../../../third_party/dart/pkg/smith
   typed_data:
     path: ../../../third_party/dart/third_party/pkg/typed_data
diff --git a/testing/skia_gold_client/test/skia_gold_client_test.dart b/testing/skia_gold_client/test/skia_gold_client_test.dart
new file mode 100644
index 0000000000000..1d094d0a48e94
--- /dev/null
+++ b/testing/skia_gold_client/test/skia_gold_client_test.dart
@@ -0,0 +1,619 @@
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io' as io;
+import 'dart:typed_data';
+
+import 'package:litetest/litetest.dart';
+import 'package:path/path.dart' as p;
+import 'package:process_fakes/process_fakes.dart';
+import 'package:skia_gold_client/skia_gold_client.dart';
+
+void main() {
+  /// A mock commit hash that is used to simulate a successful git call.
+  const String mockCommitHash = '1234567890abcdef';
+
+  /// Simulating what a presubmit environment would look like.
+  const Map<String, String> presubmitEnv = <String, String>{
+    'GOLDCTL': 'python tools/goldctl.py',
+    'GOLD_TRYJOB': 'flutter/engine/1234567890',
+    'LOGDOG_STREAM_PREFIX': 'buildbucket/cr-buildbucket.appspot.com/1234567890/+/logdog',
+    'LUCI_CONTEXT': '{}',
+  };
+
+  /// Simulating what a postsubmit environment would look like.
+  const Map<String, String> postsubmitEnv = <String, String>{
+    'GOLDCTL': 'python tools/goldctl.py',
+    'LOGDOG_STREAM_PREFIX': 'buildbucket/cr-buildbucket.appspot.com/1234567890/+/logdog',
+    'LUCI_CONTEXT': '{}'
+  };
+
+  /// Simulating what a local environment would look like.
+  const Map<String, String> localEnv = <String, String>{};
+
+  /// Creates a [SkiaGoldClient] with the given [dimensions] and [verbose] flag.
+  ///
+  /// Optionally, the [onRun] function can be provided to handle the execution
+  /// of the command-line tool. If not provided, it throws an
+  /// [UnsupportedError] by default.
+  ///
+  /// Side-effects of the client can be observed through the test fixture.
+  SkiaGoldClient createClient(
+    _TestFixture fixture, {
+    required Map<String, String> environment,
+    Map<String, String>? dimensions,
+    bool verbose = false,
+    io.ProcessResult Function(List<String> command) onRun = _runUnhandled,
+  }) {
+    return SkiaGoldClient(
+      fixture.workDirectory,
+      dimensions: dimensions,
+      httpClient: fixture.httpClient,
+      processManager: FakeProcessManager(
+        onRun: onRun,
+      ),
+      verbose: verbose,
+      stderr: fixture.outputSink,
+      environment: environment,
+    );
+  }
+
+  /// Creates a `temp/auth_opt.json` file in the working directory.
+  ///
+  /// This simulates what the goldctl tool does when it runs.
+  void createAuthOptDotJson(String workDirectory) {
+    final io.File authOptDotJson = io.File(p.join(workDirectory, 'temp', 'auth_opt.json'));
+    authOptDotJson.createSync(recursive: true);
+    authOptDotJson.writeAsStringSync('{"GSUtil": false}');
+  }
+
+  test('fails if GOLDCTL is not set', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: localEnv,
+      );
+      try {
+        await client.auth();
+        fail('auth should fail if GOLDCTL is not set');
+      } catch (error) {
+        expect('$error', contains('GOLDCTL is not set'));
+      }
+    } finally {
+      fixture.dispose();
+    }
+  });
+
+  test('auth executes successfully', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: presubmitEnv,
+        onRun: (List<String> command) {
+          expect(command, <String>[
+            'python tools/goldctl.py',
+            'auth',
+            '--work-dir',
+            p.join(fixture.workDirectory.path, 'temp'),
+            '--luci',
+          ]);
+          createAuthOptDotJson(fixture.workDirectory.path);
+          return io.ProcessResult(0, 0, '', '');
+        },
+      );
+      await client.auth();
+    } finally {
+      fixture.dispose();
+    }
+  });
+
+  test('auth is only invoked once per instance', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      int callsToGoldctl = 0;
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: presubmitEnv,
+        onRun: (List<String> command) {
+          callsToGoldctl++;
+          expect(command, <String>[
+            'python tools/goldctl.py',
+            'auth',
+            '--work-dir',
+            p.join(fixture.workDirectory.path, 'temp'),
+            '--luci',
+          ]);
+          createAuthOptDotJson(fixture.workDirectory.path);
+          return io.ProcessResult(0, 0, '', '');
+        },
+      );
+
+      await client.auth();
+      await client.auth();
+      expect(callsToGoldctl, 1);
+    } finally {
+      fixture.dispose();
+    }
+  });
+
+  test('auth executes successfully with verbose logging', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: presubmitEnv,
+        verbose: true,
+        onRun: (List<String> command) {
+          expect(command, <String>[
+            'python tools/goldctl.py',
+            'auth',
+            '--verbose',
+            '--work-dir',
+            p.join(fixture.workDirectory.path, 'temp'),
+            '--luci',
+          ]);
+          return io.ProcessResult(0, 0, 'stdout', 'stderr');
+        },
+      );
+
+      await client.auth();
+      expect(fixture.outputSink.toString(), contains('stdout:\nstdout'));
+      expect(fixture.outputSink.toString(), contains('stderr:\nstderr'));
+    } finally {
+      fixture.dispose();
+    }
+  });
+
+  test('auth fails', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: presubmitEnv,
+        onRun: (List<String> command) {
+          return io.ProcessResult(1, 0, '', 'error-text');
+        },
+      );
+
+      try {
+        await client.auth();
+      } catch (error) {
+        expect('$error', contains('Skia Gold authorization failed.'));
+        expect('$error', contains('error-text'));
+      }
+    } finally {
+      fixture.dispose();
+    }
+  });
+
+  test('addImg [pre-submit] executes successfully', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: presubmitEnv,
+        onRun: (List<String> command) {
+          if (command case ['git', ...]) {
+            return io.ProcessResult(0, 0, mockCommitHash, '');
+          }
+          if (command case ['python tools/goldctl.py', 'imgtest', 'init', ...]) {
+            return io.ProcessResult(0, 0, '', '');
+          }
+          expect(command, <String>[
+            'python tools/goldctl.py',
+            'imgtest',
+            'add',
+            '--work-dir',
+            p.join(fixture.workDirectory.path, 'temp'),
+            '--test-name',
+            'test-name',
+            '--png-file',
+            p.join(fixture.workDirectory.path, 'temp', 'golden.png'),
+            '--add-test-optional-key',
+            'image_matching_algorithm:fuzzy',
+            '--add-test-optional-key',
+            'fuzzy_max_different_pixels:10',
+            '--add-test-optional-key',
+            'fuzzy_pixel_delta_threshold:0',
+          ]);
+          return io.ProcessResult(0, 0, '', '');
+        },
+      );
+
+      await client.addImg(
+        'test-name.foo',
+        io.File(p.join(fixture.workDirectory.path, 'temp', 'golden.png')),
+        screenshotSize: 1000,
+      );
+    } finally {
+      fixture.dispose();
+    }
+  });
+
+  test('addImg [pre-submit] executes successfully with verbose logging', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: presubmitEnv,
+        verbose: true,
+        onRun: (List<String> command) {
+          if (command case ['git', ...]) {
+            return io.ProcessResult(0, 0, mockCommitHash, '');
+          }
+          if (command case ['python tools/goldctl.py', 'imgtest', 'init', ...]) {
+            return io.ProcessResult(0, 0, '', '');
+          }
+          expect(command, <String>[
+            'python tools/goldctl.py',
+            'imgtest',
+            'add',
+            '--verbose',
+            '--work-dir',
+            p.join(fixture.workDirectory.path, 'temp'),
+            '--test-name',
+            'test-name',
+            '--png-file',
+            p.join(fixture.workDirectory.path, 'temp', 'golden.png'),
+            '--add-test-optional-key',
+            'image_matching_algorithm:fuzzy',
+            '--add-test-optional-key',
+            'fuzzy_max_different_pixels:10',
+            '--add-test-optional-key',
+            'fuzzy_pixel_delta_threshold:0',
+          ]);
+          return io.ProcessResult(0, 0, 'stdout', 'stderr');
+        },
+      );
+
+      await client.addImg(
+        'test-name.foo',
+        io.File(p.join(fixture.workDirectory.path, 'temp', 'golden.png')),
+        screenshotSize: 1000,
+      );
+
+      expect(fixture.outputSink.toString(), contains('stdout:\nstdout'));
+      expect(fixture.outputSink.toString(), contains('stderr:\nstderr'));
+    } finally {
+      fixture.dispose();
+    }
+  });
+
+  // A success case (exit code 0) with a message of "Untriaged" is OK.
+  test('addImg [pre-submit] succeeds but has an untriaged image', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: presubmitEnv,
+        onRun: (List<String> command) {
+          if (command case ['git', ...]) {
+            return io.ProcessResult(0, 0, mockCommitHash, '');
+          }
+          if (command case ['python tools/goldctl.py', 'imgtest', 'init', ...]) {
+            return io.ProcessResult(0, 0, '', '');
+          }
+          expect(command, <String>[
+            'python tools/goldctl.py',
+            'imgtest',
+            'add',
+            '--work-dir',
+            p.join(fixture.workDirectory.path, 'temp'),
+            '--test-name',
+            'test-name',
+            '--png-file',
+            p.join(fixture.workDirectory.path, 'temp', 'golden.png'),
+            '--add-test-optional-key',
+            'image_matching_algorithm:fuzzy',
+            '--add-test-optional-key',
+            'fuzzy_max_different_pixels:10',
+            '--add-test-optional-key',
+            'fuzzy_pixel_delta_threshold:0',
+          ]);
+          // Intentionally returning a non-zero exit code.
+          return io.ProcessResult(0, 1, 'Untriaged', '');
+        },
+      );
+
+      await client.addImg(
+        'test-name.foo',
+        io.File(p.join(fixture.workDirectory.path, 'temp', 'golden.png')),
+        screenshotSize: 1000,
+      );
+    } finally {
+      fixture.dispose();
+    }
+  });
+
+  test('addImg [pre-submit] fails due to an unexpected error', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: presubmitEnv,
+        onRun: (List<String> command) {
+          if (command case ['git', ...]) {
+            return io.ProcessResult(0, 0, mockCommitHash, '');
+          }
+          if (command case ['python tools/goldctl.py', 'imgtest', 'init', ...]) {
+            return io.ProcessResult(0, 0, '', '');
+          }
+          return io.ProcessResult(1, 0, '', 'error-text');
+        },
+      );
+
+      try {
+        await client.addImg(
+          'test-name.foo',
+          io.File(p.join(fixture.workDirectory.path, 'temp', 'golden.png')),
+          screenshotSize: 1000,
+        );
+      } catch (error) {
+        expect('$error', contains('Skia Gold image test failed.'));
+        expect('$error', contains('error-text'));
+      }
+    } finally {
+      fixture.dispose();
+    }
+  });
+
+  test('addImg [post-submit] executes successfully', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: postsubmitEnv,
+        onRun: (List<String> command) {
+          if (command case ['git', ...]) {
+            return io.ProcessResult(0, 0, mockCommitHash, '');
+          }
+          if (command case ['python tools/goldctl.py', 'imgtest', 'init', ...]) {
+            return io.ProcessResult(0, 0, '', '');
+          }
+          expect(command, <String>[
+            'python tools/goldctl.py',
+            'imgtest',
+            'add',
+            '--work-dir',
+            p.join(fixture.workDirectory.path, 'temp'),
+            '--test-name',
+            'test-name',
+            '--png-file',
+            p.join(fixture.workDirectory.path, 'temp', 'golden.png'),
+            '--passfail',
+            '--add-test-optional-key',
+            'image_matching_algorithm:fuzzy',
+            '--add-test-optional-key',
+            'fuzzy_max_different_pixels:10',
+            '--add-test-optional-key',
+            'fuzzy_pixel_delta_threshold:0',
+          ]);
+          return io.ProcessResult(0, 0, '', '');
+        },
+      );
+
+      await client.addImg(
+        'test-name.foo',
+        io.File(p.join(fixture.workDirectory.path, 'temp', 'golden.png')),
+        screenshotSize: 1000,
+      );
+    } finally {
+      fixture.dispose();
+    }
+  });
+
+  test('addImg [post-submit] executes successfully with verbose logging', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: postsubmitEnv,
+        verbose: true,
+        onRun: (List<String> command) {
+          if (command case ['git', ...]) {
+            return io.ProcessResult(0, 0, mockCommitHash, '');
+          }
+          if (command case ['python tools/goldctl.py', 'imgtest', 'init', ...]) {
+            return io.ProcessResult(0, 0, '', '');
+          }
+          expect(command, <String>[
+            'python tools/goldctl.py',
+            'imgtest',
+            'add',
+            '--verbose',
+            '--work-dir',
+            p.join(fixture.workDirectory.path, 'temp'),
+            '--test-name',
+            'test-name',
+            '--png-file',
+            p.join(fixture.workDirectory.path, 'temp', 'golden.png'),
+            '--passfail',
+            '--add-test-optional-key',
+            'image_matching_algorithm:fuzzy',
+            '--add-test-optional-key',
+            'fuzzy_max_different_pixels:10',
+            '--add-test-optional-key',
+            'fuzzy_pixel_delta_threshold:0',
+          ]);
+          return io.ProcessResult(0, 0, 'stdout', 'stderr');
+        },
+      );
+
+      await client.addImg(
+        'test-name.foo',
+        io.File(p.join(fixture.workDirectory.path, 'temp', 'golden.png')),
+        screenshotSize: 1000,
+      );
+
+      expect(fixture.outputSink.toString(), contains('stdout:\nstdout'));
+      expect(fixture.outputSink.toString(), contains('stderr:\nstderr'));
+    } finally {
+      fixture.dispose();
+    }
+  });
+
+  test('addImg [post-submit] fails due to an unapproved image', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: postsubmitEnv,
+        onRun: (List<String> command) {
+          if (command case ['git', ...]) {
+            return io.ProcessResult(0, 0, mockCommitHash, '');
+          }
+          if (command case ['python tools/goldctl.py', 'imgtest', 'init', ...]) {
+            return io.ProcessResult(0, 0, '', '');
+          }
+          return io.ProcessResult(1, 0, '', 'error-text');
+        },
+      );
+
+      try {
+        await client.addImg(
+          'test-name.foo',
+          io.File(p.join(fixture.workDirectory.path, 'temp', 'golden.png')),
+          screenshotSize: 1000,
+        );
+      } catch (error) {
+        expect('$error', contains('Skia Gold image test failed.'));
+        expect('$error', contains('error-text'));
+      }
+    } finally {
+      fixture.dispose();
+    }
+  });
+
+  test('getExpectationsForTest returns the latest positive digest', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: presubmitEnv,
+        onRun: (List<String> command) {
+          expect(command, <String>[
+            'python tools/goldctl.py',
+            'imgtest',
+            'get',
+            '--work-dir',
+            p.join(fixture.workDirectory.path, 'temp'),
+            '--test-name',
+            'test-name',
+          ]);
+          return io.ProcessResult(0, 0, '{"digest":"digest"}', '');
+        },
+      );
+
+      final String hash = client.getTraceID('test-name');
+      fixture.httpClient.setJsonResponse(
+        Uri.parse('https://flutter-engine-gold.skia.org/json/v2/latestpositivedigest/$hash'),
+        <String, Object?>{
+          'digest': 'digest',
+        },
+      );
+
+      final String? digest = await client.getExpectationForTest('test-name');
+      expect(digest, 'digest');
+    } finally {
+      fixture.dispose();
+    }
+  });
+}
+
+final class _TestFixture {
+  _TestFixture();
+
+  final io.Directory workDirectory = io.Directory.systemTemp.createTempSync('skia_gold_client_test');
+  final _FakeHttpClient httpClient = _FakeHttpClient();
+  final StringSink outputSink = StringBuffer();
+
+  void dispose() {
+    workDirectory.deleteSync(recursive: true);
+  }
+}
+
+io.ProcessResult _runUnhandled(List<String> command) {
+  throw UnimplementedError('Unhandled run: ${command.join(' ')}');
+}
+
+/// An  in-memory fake of [io.HttpClient] that allows [getUrl] to be mocked.
+///
+/// This class is used to simulate a response from the server.
+///
+/// Any other methods called on this class will throw a [NoSuchMethodError].
+final class _FakeHttpClient implements io.HttpClient {
+  final Map<Uri, Object?> _expectedResponses = <Uri, Object?>{};
+
+  /// Sets an expected response for the given [request] to [jsonEncodableValue].
+  ///
+  /// This method is used to simulate a response from the server.
+  void setJsonResponse(Uri request, Object? jsonEncodableValue) {
+    _expectedResponses[request] = jsonEncodableValue;
+  }
+
+  @override
+  Future<io.HttpClientRequest> getUrl(Uri url) async {
+    final Object? response = _expectedResponses[url];
+    if (response == null) {
+      throw StateError('No request expected for $url');
+    }
+    return _FakeHttpClientRequest.withJsonResponse(response);
+  }
+
+  @override
+  Object? noSuchMethod(Invocation invocation) {
+    return super.noSuchMethod(invocation);
+  }
+}
+
+final class _FakeHttpClientRequest implements io.HttpClientRequest {
+  factory _FakeHttpClientRequest.withJsonResponse(Object? jsonResponse) {
+    final Uint8List bytes = utf8.encoder.convert(jsonEncode(jsonResponse));
+    return _FakeHttpClientRequest._(_FakeHttpClientResponse(bytes));
+  }
+
+  _FakeHttpClientRequest._(this._response);
+
+  final io.HttpClientResponse _response;
+
+  @override
+  Future<io.HttpClientResponse> close() async {
+    return _response;
+  }
+
+  @override
+  Object? noSuchMethod(Invocation invocation) {
+    return super.noSuchMethod(invocation);
+  }
+}
+
+final class _FakeHttpClientResponse extends Stream<List<int>>
+    implements io.HttpClientResponse {
+  _FakeHttpClientResponse(this._bytes);
+
+  final Uint8List _bytes;
+
+  @override
+  StreamSubscription<List<int>> listen(
+    void Function(List<int> event)? onData, {
+    Function? onError,
+    void Function()? onDone,
+    bool? cancelOnError,
+  }) {
+    return Stream<List<int>>.fromIterable(<List<int>>[_bytes]).listen(
+      onData,
+      onError: onError,
+      onDone: onDone,
+      cancelOnError: cancelOnError,
+    );
+  }
+
+  @override
+  int get statusCode => 200;
+
+  @override
+  Object? noSuchMethod(Invocation invocation) {
+    return super.noSuchMethod(invocation);
+  }
+}
diff --git a/tools/golden_tests_harvester/bin/golden_tests_harvester.dart b/tools/golden_tests_harvester/bin/golden_tests_harvester.dart
index 62a5f4525d856..021d97505cb9a 100644
--- a/tools/golden_tests_harvester/bin/golden_tests_harvester.dart
+++ b/tools/golden_tests_harvester/bin/golden_tests_harvester.dart
@@ -40,26 +40,11 @@ class FakeSkiaGoldClient implements SkiaGoldClient {
     Logger.instance.log('auth dimensions:${dimensions ?? 'null'}');
   }
 
-  @override
-  String cleanTestName(String fileName) {
-    throw UnimplementedError();
-  }
-
-  @override
-  List<String> getCIArguments() {
-    throw UnimplementedError();
-  }
-
   @override
   Future<String?> getExpectationForTest(String testName) {
     throw UnimplementedError();
   }
 
-  @override
-  Future<List<int>> getImageBytes(String imageHash) {
-    throw UnimplementedError();
-  }
-
   @override
   String getTraceID(String testName) {
     throw UnimplementedError();