Add google_sign_in_web plugin.

packages/google_sign_in/google_sign_in_web/CHANGELOG.md

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+## 0.8.0
+
+* Flutter for web initial release

packages/google_sign_in/google_sign_in_web/LICENSE

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+Copyright 2016, the Flutter project authors. All rights reserved.
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+    * Neither the name of Google Inc. nor the names of its
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

packages/google_sign_in/google_sign_in_web/README.md

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+# google_sign_in_web
+
+The web implementation of [google_sign_in](https://pub.dev/google_sign_in/google_sign_in)
+
+## Usage
+
+### Import the package
+To use this plugin, follow the [plugin installation instructions](https://pub.dartlang.org/packages/google_sign_in#pub-pkg-tab-installing).
+
+Remember that for web plugins you need to depend both on the "native" version that provides the Dart interface that you'll use in your app), and the "web" version, that provides the implementation of the plugin for the web platform.
+
+This is what the above means to your `pubspec.yaml`:
+
+```
+...
+dependencies:
+  ...
+  google_sign_in: ^4.0.14
+  google_sign_in_web: ^0.8.0
+  ...
+...
+```
+
+### Web integration
+
+First, go through the instructions [here](https://developers.google.com/identity/sign-in/web/sign-in#before_you_begin) to create your Google Sign-In OAuth client ID.
+
+On your `web/index.html` file, add the following `meta` tag, somewhere in the
+`head` of the document:
+
+```
+  <meta name="google-signin-client_id" content="YOUR_GOOGLE_SIGN_IN_OAUTH_CLIENT_ID.apps.googleusercontent.com">
+```
+
+Read the rest of the instructions if you need to add extra APIs (like Google People API).
+
+
+### Use the plugin
+Add the following import to your Dart code:
+
+```dart
+import 'package:google_sign_in/google_sign_in.dart';
+```
+
+Initialize GoogleSignIn with the scopes you want:
+
+```dart
+GoogleSignIn _googleSignIn = GoogleSignIn(
+  scopes: [
+    'email',
+    'https://www.googleapis.com/auth/contacts.readonly',
+  ],
+);
+```
+[Full list of available scopes](https://developers.google.com/identity/protocols/googlescopes).
+
+You can now use the `GoogleSignIn` class to authenticate in your Dart code, e.g.
+
+```dart
+Future<void> _handleSignIn() async {
+  try {
+    await _googleSignIn.signIn();
+  } catch (error) {
+    print(error);
+  }
+}
+```
+
+## Example
+
+Find the example wiring in the [Google sign-in example application](https://github.com/flutter/plugins/blob/master/packages/google_sign_in/google_sign_in/example/lib/main.dart).
+
+## API details
+
+See the [google_sign_in.dart](https://github.com/flutter/plugins/blob/master/packages/google_sign_in/google_sign_in/lib/google_sign_in.dart) for more API details.
+
+## Issues and feedback
+
+Please file [issues](https://github.com/flutter/flutter/issues/new)
+to send feedback or report a bug. Thank you!

packages/google_sign_in/google_sign_in_web/analysis_options.yaml

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+# Exclude auto-generated files from analysis/linting
+analyzer:
+  exclude:
+    - "lib/src/generated/**"
+

packages/google_sign_in/google_sign_in_web/lib/google_sign_in_web.dart

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+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:html' as html;
+
+import 'package:google_sign_in_platform_interface/google_sign_in_platform_interface.dart';
+import 'package:flutter_web_plugins/flutter_web_plugins.dart';
+import 'package:js/js.dart';
+import 'package:meta/meta.dart';
+
+import 'src/generated/gapiauth2.dart' as auth2;
+// TODO: Remove once this lands https://github.com/dart-lang/language/issues/671
+import 'src/generated/gapiauth2.dart' show GoogleAuthExtensions;
+import 'src/load_gapi.dart' as gapi;
+import 'src/utils.dart' show gapiUserToPluginUserData;
+
+const String _kClientIdMetaSelector = 'meta[name=google-signin-client_id]';
+const String _kClientIdAttributeName = 'content';
+
+@visibleForTesting
+String gapiUrl = 'https://apis.google.com/js/platform.js';
+
+/// Implementation of the google_sign_in plugin for Web
+class GoogleSignInPlugin extends GoogleSignInPlatform {
+  GoogleSignInPlugin() {
+    _autoDetectedClientId = html
+        .querySelector(_kClientIdMetaSelector)
+        ?.getAttribute(_kClientIdAttributeName);
+
+    _isGapiInitialized = gapi.inject(gapiUrl).then((_) => gapi.init());
+  }
+
+  Future<void> _isGapiInitialized;
+
+  @visibleForTesting
+  Future<void> get initialized => _isGapiInitialized;
+
+  String _autoDetectedClientId;
+  FutureOr<auth2.GoogleUser> _lastSeenUser;
+
+  static void registerWith(Registrar registrar) {
+    GoogleSignInPlatform.instance = GoogleSignInPlugin();
+  }
+
+  @override
+  Future<void> init(
+      {@required String hostedDomain,
+      List<String> scopes = const <String>[],
+      SignInOption signInOption = SignInOption.standard,
+      String clientId}) async {
+    final String appClientId = clientId ?? _autoDetectedClientId;
+    assert(
+        appClientId != null,
+        'ClientID not set. Either set it on a '
+        '<meta name="google-signin-client_id" content="CLIENT_ID" /> tag,'
+        ' or pass clientId when calling init()');
+
+    assert(
+        !scopes.any((String scope) => scope.contains(' ')),
+        'OAuth 2.0 Scopes for Google APIs can\'t contain spaces.'
+        'Check https://developers.google.com/identity/protocols/googlescopes '
+        'for a list of valid OAuth 2.0 scopes.');
+
+    await initialized;
+
+    final auth2.GoogleAuth auth = auth2.init(auth2.ClientConfig(
+      hosted_domain: hostedDomain,
+      // The js lib wants a space-separated list of values
+      scope: scopes.join(' '),
+      client_id: appClientId,
+    ));
+
+    // Subscribe to changes in the auth instance returned by init,
+    // and cache the _lastSeenUser as we get notified of new values.
+    final Completer<auth2.GoogleUser> initUserCompleter =
+        Completer<auth2.GoogleUser>();
+
+    auth.currentUser.listen(allowInterop((auth2.GoogleUser nextUser) {
+      if (!initUserCompleter.isCompleted) {
+        initUserCompleter.complete(nextUser);
+      } else {
+        _lastSeenUser = nextUser;
+      }
+    }));
+    _lastSeenUser = initUserCompleter.future;
+
+    return null;
+  }
+
+  @override
+  Future<GoogleSignInUserData> signInSilently() async {
+    await initialized;
+
+    return gapiUserToPluginUserData(await _lastSeenUser);
+  }
+
+  @override
+  Future<GoogleSignInUserData> signIn() async {
+    await initialized;
+
+    return gapiUserToPluginUserData(await auth2.getAuthInstance().signIn());
+  }
+
+  @override
+  Future<GoogleSignInTokenData> getTokens(
+      {@required String email, bool shouldRecoverAuth}) async {
+    await initialized;
+
+    final auth2.GoogleUser currentUser =
+        auth2.getAuthInstance()?.currentUser?.get();
+    final auth2.AuthResponse response = currentUser.getAuthResponse();
+
+    return GoogleSignInTokenData(
+        idToken: response.id_token, accessToken: response.access_token);
+  }
+
+  @override
+  Future<void> signOut() async {
+    await initialized;
+
+    return auth2.getAuthInstance().signOut();
+  }
+
+  @override
+  Future<void> disconnect() async {
+    await initialized;
+
+    final auth2.GoogleUser currentUser =
+        auth2.getAuthInstance()?.currentUser?.get();
+    return currentUser.disconnect();
+  }
+
+  @override
+  Future<bool> isSignedIn() async {
+    await initialized;
+
+    final auth2.GoogleUser currentUser =
+        auth2.getAuthInstance()?.currentUser?.get();
+    return currentUser.isSignedIn();
+  }
+
+  @override
+  Future<void> clearAuthCache({String token}) async {
+    await initialized;
+
+    _lastSeenUser = null;
+    return auth2.getAuthInstance().disconnect();
+  }
+}