From c276997884651065f4a925675ac7ae00f5cd2542 Mon Sep 17 00:00:00 2001
From: Jackson Gardner <jacksongardner@google.com>
Date: Wed, 5 Jun 2024 12:17:34 -0700
Subject: [PATCH 1/4] Fixes a few issues in flutter_js

This addresses a couple things:
https://github.com/flutter/flutter/issues/147610 (Treat `auto` renderer properly)
https://github.com/flutter/flutter/issues/149443 (Add an entrypoint base url config option)

This also adds a `useLocalCanvasKit` configuration option on the build
config, which the flutter tool can use to fix https://github.com/flutter/flutter/issues/148713
---
 lib/web_ui/dev/test_platform.dart             |  2 +-
 lib/web_ui/flutter_js/sources.gni             |  2 +-
 lib/web_ui/flutter_js/src/base_uri.js         | 17 -------
 lib/web_ui/flutter_js/src/canvaskit_loader.js | 15 +++---
 .../flutter_js/src/entrypoint_loader.js       | 16 ++++---
 lib/web_ui/flutter_js/src/loader.js           | 22 +++++++--
 .../flutter_js/src/service_worker_loader.js   |  4 +-
 lib/web_ui/flutter_js/src/skwasm_loader.js    | 18 +++----
 lib/web_ui/flutter_js/src/types.d.ts          |  3 ++
 lib/web_ui/flutter_js/src/utils.js            | 48 +++++++++++++++++++
 10 files changed, 100 insertions(+), 47 deletions(-)
 delete mode 100644 lib/web_ui/flutter_js/src/base_uri.js
 create mode 100644 lib/web_ui/flutter_js/src/utils.js

diff --git a/lib/web_ui/dev/test_platform.dart b/lib/web_ui/dev/test_platform.dart
index 078b18ee86322..34954329d30f7 100644
--- a/lib/web_ui/dev/test_platform.dart
+++ b/lib/web_ui/dev/test_platform.dart
@@ -562,6 +562,7 @@ class BrowserPlatform extends PlatformPlugin {
 </script>
 <script>
   _flutter.buildConfig = {
+    useLocalCanvaskit: true,
     builds: [
       $buildConfigsString
     ]
@@ -571,7 +572,6 @@ class BrowserPlatform extends PlatformPlugin {
 <script>
   _flutter.loader.load({
     config: {
-      canvasKitBaseUrl: "/canvaskit/",
       // Some of our tests rely on color emoji
       useColorEmoji: true,
       canvasKitVariant: "${getCanvasKitVariant()}",
diff --git a/lib/web_ui/flutter_js/sources.gni b/lib/web_ui/flutter_js/sources.gni
index 11da02b200455..7cdfbb5188dee 100644
--- a/lib/web_ui/flutter_js/sources.gni
+++ b/lib/web_ui/flutter_js/sources.gni
@@ -3,7 +3,6 @@
 # found in the LICENSE file.
 
 flutter_js_source_list = [
-  "src/base_uri.js",
   "src/browser_environment.js",
   "src/canvaskit_loader.js",
   "src/entrypoint_loader.js",
@@ -13,6 +12,7 @@ flutter_js_source_list = [
   "src/service_worker_loader.js",
   "src/skwasm_loader.js",
   "src/trusted_types.js",
+  "src/utils.js",
 
   "src/types.d.ts",
 ]
diff --git a/lib/web_ui/flutter_js/src/base_uri.js b/lib/web_ui/flutter_js/src/base_uri.js
deleted file mode 100644
index bfca8d341bde7..0000000000000
--- a/lib/web_ui/flutter_js/src/base_uri.js
+++ /dev/null
@@ -1,17 +0,0 @@
-// Copyright 2013 The Flutter Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-export const baseUri = ensureTrailingSlash(getBaseURI());
-
-function getBaseURI() {
-  const base = document.querySelector("base");
-  return (base && base.getAttribute("href")) || "";
-}
-
-function ensureTrailingSlash(uri) {
-  if (uri === "") {
-    return uri;
-  }
-  return uri.endsWith("/") ? uri : `${uri}/`;
-}
diff --git a/lib/web_ui/flutter_js/src/canvaskit_loader.js b/lib/web_ui/flutter_js/src/canvaskit_loader.js
index 39a696a6a8686..64dc6734ec273 100644
--- a/lib/web_ui/flutter_js/src/canvaskit_loader.js
+++ b/lib/web_ui/flutter_js/src/canvaskit_loader.js
@@ -3,8 +3,9 @@
 // found in the LICENSE file.
 
 import { createWasmInstantiator } from "./instantiate_wasm.js";
+import { joinPathSegments } from "./utils.js";
 
-export const loadCanvasKit = (deps, config, browserEnvironment, engineRevision) => {
+export const loadCanvasKit = (deps, config, browserEnvironment, canvasKitBaseUrl) => {
   if (window.flutterCanvasKit) {
     // The user has set this global variable ahead of time, so we just return that.
     return Promise.resolve(window.flutterCanvasKit);
@@ -15,21 +16,21 @@ export const loadCanvasKit = (deps, config, browserEnvironment, engineRevision)
       throw "Chromium CanvasKit variant specifically requested, but unsupported in this browser";
     }
     const useChromiumCanvasKit = supportsChromiumCanvasKit && (config.canvasKitVariant !== "full");
-    let baseUrl = config.canvasKitBaseUrl ?? `https://www.gstatic.com/flutter-canvaskit/${engineRevision}/`;
+    let baseUrl = canvasKitBaseUrl;
     if (useChromiumCanvasKit) {
-      baseUrl = `${baseUrl}chromium/`;
+      baseUrl = joinPathSegments(baseUrl, "chromium");
     }
-    let canvasKitUrl = `${baseUrl}canvaskit.js`;
+    let canvasKitUrl = joinPathSegments(baseUrl, "canvaskit.js");
     if (deps.flutterTT.policy) {
       canvasKitUrl = deps.flutterTT.policy.createScriptURL(canvasKitUrl);
     }
-    const wasmInstantiator = createWasmInstantiator(`${baseUrl}canvaskit.wasm`);
+    const wasmInstantiator = createWasmInstantiator(joinPathSegments(baseUrl, "canvaskit.wasm"));
     const script = document.createElement("script");
     script.src = canvasKitUrl;
     if (config.nonce) {
       script.nonce = config.nonce;
     }
-    script.addEventListener('load', async () => {
+    script.addEventListener("load", async () => {
       try {
         const canvasKit = await CanvasKitInit({
           instantiateWasm: wasmInstantiator,
@@ -40,7 +41,7 @@ export const loadCanvasKit = (deps, config, browserEnvironment, engineRevision)
         reject(e);
       }
     });
-    script.addEventListener('error', reject);
+    script.addEventListener("error", reject);
     document.head.appendChild(script);
   });
   return window.flutterCanvasKitLoaded;
diff --git a/lib/web_ui/flutter_js/src/entrypoint_loader.js b/lib/web_ui/flutter_js/src/entrypoint_loader.js
index db8b33159dc48..3e789c05e718b 100644
--- a/lib/web_ui/flutter_js/src/entrypoint_loader.js
+++ b/lib/web_ui/flutter_js/src/entrypoint_loader.js
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import { baseUri } from "./base_uri.js";
+import { baseUri, joinPathSegments } from "./utils.js";
 
 /**
  * Handles injecting the main Flutter web entrypoint (main.dart.js), and notifying
@@ -37,7 +37,7 @@ export class FlutterEntrypointLoader {
    * Returns undefined when an `onEntrypointLoaded` callback is supplied in `options`.
    */
   async loadEntrypoint(options) {
-    const { entrypointUrl = `${baseUri}main.dart.js`, onEntrypointLoaded, nonce } =
+    const { entrypointUrl = joinPathSegments(baseUri, "main.dart.js"), onEntrypointLoaded, nonce } =
       options || {};
     return this._loadJSEntrypoint(entrypointUrl, onEntrypointLoaded, nonce);
   }
@@ -63,11 +63,12 @@ export class FlutterEntrypointLoader {
     onEntrypointLoaded ??= (engineInitializer) => {
       engineInitializer.initializeEngine(config).then((appRunner) => appRunner.runApp())
     };
+    const { entryPointBaseUrl } = config;
     if (build.compileTarget === "dart2wasm") {
-      return this._loadWasmEntrypoint(build, deps, onEntrypointLoaded);
+      return this._loadWasmEntrypoint(build, deps, entryPointBaseUrl, onEntrypointLoaded);
     } else {
       const mainPath = build.mainJsPath ?? "main.dart.js";
-      const entrypointUrl = `${baseUri}${mainPath}`;
+      const entrypointUrl = joinPathSegments(baseUri, entryPointBaseUrl, mainPath);
       return this._loadJSEntrypoint(entrypointUrl, onEntrypointLoaded, nonce);
     }
   }
@@ -138,16 +139,17 @@ export class FlutterEntrypointLoader {
    *
    * @param {import("./types").WasmApplicationBuild} build
    * @param {*} deps
+   * @param {string} entryPointBaseUrl
    * @param {import("./types").OnEntrypointLoadedCallback} onEntrypointLoaded
    */
-  async _loadWasmEntrypoint(build, deps, onEntrypointLoaded) {
+  async _loadWasmEntrypoint(build, deps, entrypointBaseUrl, onEntrypointLoaded) {
     if (!this._scriptLoaded) {
       this._scriptLoaded = true;
 
       this._onEntrypointLoaded = onEntrypointLoaded;
       const { mainWasmPath, jsSupportRuntimePath } = build;
-      const moduleUri = `${baseUri}${mainWasmPath}`;
-      let jsSupportRuntimeUri = `${baseUri}${jsSupportRuntimePath}`;
+      const moduleUri = joinPathSegments(baseUri, entrypointBaseUrl, mainWasmPath);
+      let jsSupportRuntimeUri = joinPathSegments(baseUri, entrypointBaseUrl, jsSupportRuntimePath);
       if (this._ttPolicy != null) {
         jsSupportRuntimeUri = this._ttPolicy.createScriptURL(jsSupportRuntimeUri);
       }  
diff --git a/lib/web_ui/flutter_js/src/loader.js b/lib/web_ui/flutter_js/src/loader.js
index 3d5fb4e8ddbef..1d4e3c6c37322 100644
--- a/lib/web_ui/flutter_js/src/loader.js
+++ b/lib/web_ui/flutter_js/src/loader.js
@@ -8,6 +8,7 @@ import { FlutterServiceWorkerLoader } from './service_worker_loader.js';
 import { FlutterTrustedTypesPolicy } from './trusted_types.js';
 import { loadCanvasKit } from './canvaskit_loader.js';
 import { loadSkwasm } from './skwasm_loader.js';
+import { getDefaultCanvaskitBaseUrl } from './utils.js';
 
 /**
  * The public interface of _flutter.loader. Exposes two methods:
@@ -87,11 +88,25 @@ export class FlutterLoader {
       }
     }
 
+    /**
+     * @param {import("./types").ApplicationBuild} build
+     * @param {import("./types").WebRenderer} renderer
+     **/
+    const buildContainsRenderer = (build, renderer) => {
+      switch (build.renderer) {
+        // The "auto" build contains both canvaskit and html renderers.
+        case "auto":
+          return renderer == "canvaskit" || renderer == "html";
+        default:
+          return build.renderer == renderer;
+      }
+    }
+
     const buildIsCompatible = (build) => {
       if (build.compileTarget === "dart2wasm" && !browserEnvironment.supportsWasmGC) {
         return false;
       }
-      if (config.renderer && config.renderer != build.renderer) {
+      if (config.renderer && !buildContainsRenderer(build, config.renderer)) {
         return false;
       }
       return rendererIsCompatible(build.renderer);
@@ -112,10 +127,11 @@ export class FlutterLoader {
       });
     }
 
+    const canvasKitBaseUrl = getDefaultCanvaskitBaseUrl(config, buildConfig);
     if (build.renderer === "canvaskit") {
-      deps.canvasKit = loadCanvasKit(deps, config, browserEnvironment, buildConfig.engineRevision);
+      deps.canvasKit = loadCanvasKit(deps, config, browserEnvironment, canvasKitBaseUrl);
     } else if (build.renderer === "skwasm") {
-      deps.skwasm = loadSkwasm(deps, config, browserEnvironment, buildConfig.engineRevision);
+      deps.skwasm = loadSkwasm(deps, config, browserEnvironment, canvasKitBaseUrl);
     }
 
     // The FlutterEntrypointLoader instance could be injected as a dependency
diff --git a/lib/web_ui/flutter_js/src/service_worker_loader.js b/lib/web_ui/flutter_js/src/service_worker_loader.js
index 7c4ff3e6c39bd..20d9f25f68728 100644
--- a/lib/web_ui/flutter_js/src/service_worker_loader.js
+++ b/lib/web_ui/flutter_js/src/service_worker_loader.js
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import { baseUri } from "./base_uri.js";
+import { baseUri, joinPathSegments } from "./utils.js";
 
 /**
  * Wraps `promise` in a timeout of the given `duration` in ms.
@@ -78,7 +78,7 @@ export class FlutterServiceWorkerLoader {
     }
     const {
       serviceWorkerVersion,
-      serviceWorkerUrl = `${baseUri}flutter_service_worker.js?v=${serviceWorkerVersion}`,
+      serviceWorkerUrl = joinPathSegments(baseUri, `flutter_service_worker.js?v=${serviceWorkerVersion}`),
       timeoutMillis = 4000,
     } = settings;
     // Apply the TrustedTypes policy, if present.
diff --git a/lib/web_ui/flutter_js/src/skwasm_loader.js b/lib/web_ui/flutter_js/src/skwasm_loader.js
index 86e1b529784d3..241f4e8c81e02 100644
--- a/lib/web_ui/flutter_js/src/skwasm_loader.js
+++ b/lib/web_ui/flutter_js/src/skwasm_loader.js
@@ -3,21 +3,21 @@
 // found in the LICENSE file.
 
 import { createWasmInstantiator } from "./instantiate_wasm.js";
+import { joinPathSegments } from "./utils.js";
 
-export const loadSkwasm = (deps, config, browserEnvironment, engineRevision) => {
+export const loadSkwasm = (deps, config, browserEnvironment, baseUrl) => {
   return new Promise((resolve, reject) => {
-    const baseUrl = config.canvasKitBaseUrl ?? `https://www.gstatic.com/flutter-canvaskit/${engineRevision}/`;
-    let skwasmUrl = `${baseUrl}skwasm.js`;
+    let skwasmUrl = joinPathSegments(baseUrl, "skwasm.js");
     if (deps.flutterTT.policy) {
       skwasmUrl = deps.flutterTT.policy.createScriptURL(skwasmUrl);
     }
-    const wasmInstantiator = createWasmInstantiator(`${baseUrl}skwasm.wasm`);
+    const wasmInstantiator = createWasmInstantiator(joinPathSegments(baseUrl, "skwasm.wasm"));
     const script = document.createElement("script");
     script.src = skwasmUrl;
     if (config.nonce) {
       script.nonce = config.nonce;
     }
-    script.addEventListener('load', async () => {
+    script.addEventListener("load", async () => {
       try {
         const skwasmInstance = await skwasm({
           instantiateWasm: wasmInstantiator,
@@ -28,10 +28,10 @@ export const loadSkwasm = (deps, config, browserEnvironment, engineRevision) =>
             // make sure that the worker JS can be loaded regardless of where
             // it is hosted.
             const url = scriptDirectory + fileName;
-            if (url.endsWith('.worker.js')) {
+            if (url.endsWith(".worker.js")) {
               return URL.createObjectURL(new Blob(
-                [`importScripts('${url}');`],
-                { 'type': 'application/javascript' }));
+                [`importScripts("${url}");`],
+                { "type": "application/javascript" }));
             }
             return url;
           }
@@ -41,7 +41,7 @@ export const loadSkwasm = (deps, config, browserEnvironment, engineRevision) =>
         reject(e);
       }
     });
-    script.addEventListener('error', reject);
+    script.addEventListener("error", reject);
     document.head.appendChild(script);
   });
 }
diff --git a/lib/web_ui/flutter_js/src/types.d.ts b/lib/web_ui/flutter_js/src/types.d.ts
index 4013ddbcec264..e3cdee0515928 100644
--- a/lib/web_ui/flutter_js/src/types.d.ts
+++ b/lib/web_ui/flutter_js/src/types.d.ts
@@ -8,6 +8,7 @@ type WasmCompileTarget = "dart2wasm";
 export type CompileTarget = JSCompileTarget | WasmCompileTarget;
 
 export type WebRenderer =
+  "auto" |
   "html" |
   "canvaskit" |
   "skwasm";
@@ -32,6 +33,7 @@ export type ApplicationBuild = JSApplicationBuild | WasmApplicationBuild;
 export interface BuildConfig {
   serviceWorkerVersion: string;
   engineRevision: string;
+  useLocalCanvasKit: bool?;
   builds: ApplicationBuild[];
 }
 
@@ -54,6 +56,7 @@ export interface FlutterConfiguration {
   renderer: WebRenderer?;
   hostElement: HtmlElement?;
   fontFallbackBaseUrl: string?;
+  entryPointBaseUrl: string?;
 }
 
 export interface ServiceWorkerSettings {
diff --git a/lib/web_ui/flutter_js/src/utils.js b/lib/web_ui/flutter_js/src/utils.js
new file mode 100644
index 0000000000000..f098a08135b3c
--- /dev/null
+++ b/lib/web_ui/flutter_js/src/utils.js
@@ -0,0 +1,48 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+export const baseUri = getBaseURI();
+
+function getBaseURI() {
+  const base = document.querySelector("base");
+  return (base && base.getAttribute("href")) || "";
+}
+
+export function joinPathSegments(...segments) {
+  return segments.map((segment, i) => {
+    if (!segment) {
+      return "";
+    }
+
+    if (i === 0) {
+      return stripRightSlash(segment);
+    } else {
+      return stripLeftSlash(stripRightSlash(segment));
+    }
+  }).filter(x => x.length).join('/')
+}
+
+function stripLeftSlash(s) {
+  return s.startsWith('/') ? s.substring(1) : s;
+}
+
+function stripRightSlash(s) {
+  return s.endsWith('/') ? s.substring(0, s.length - 1) : s;
+}
+
+/**
+ * Calculates the proper base URL for CanvasKit/Skwasm assets.
+ * 
+ * @param {import("./types").FlutterConfiguration} config
+ * @param {import("./types").BuildConfig} buildConfig
+ */
+export function getDefaultCanvaskitBaseUrl(config, buildConfig) {
+  if (config.canvasKitBaseUrl) {
+    return config.canvasKitBaseUrl;
+  }
+  if (buildConfig.engineRevision && !buildConfig.useLocalCanvasKit) {
+    return joinPathSegments('https://www.gstatic.com/flutter-canvaskit', buildConfig.engineRevision);
+  }
+  return '/canvaskit';
+}
\ No newline at end of file

From 2e38d2b474995730bbe97c39566daa719ec47d60 Mon Sep 17 00:00:00 2001
From: Jackson Gardner <jacksongardner@google.com>
Date: Wed, 5 Jun 2024 12:41:04 -0700
Subject: [PATCH 2/4] Add trailing newline.

---
 lib/web_ui/flutter_js/src/utils.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/web_ui/flutter_js/src/utils.js b/lib/web_ui/flutter_js/src/utils.js
index f098a08135b3c..8f52db6217855 100644
--- a/lib/web_ui/flutter_js/src/utils.js
+++ b/lib/web_ui/flutter_js/src/utils.js
@@ -45,4 +45,4 @@ export function getDefaultCanvaskitBaseUrl(config, buildConfig) {
     return joinPathSegments('https://www.gstatic.com/flutter-canvaskit', buildConfig.engineRevision);
   }
   return '/canvaskit';
-}
\ No newline at end of file
+}

From 1463f3fa9ffa636db52e20ddf2f25034e8e7a6ba Mon Sep 17 00:00:00 2001
From: Jackson Gardner <jacksongardner@google.com>
Date: Wed, 5 Jun 2024 12:58:44 -0700
Subject: [PATCH 3/4] Fix licenses file.

---
 ci/licenses_golden/licenses_flutter | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/ci/licenses_golden/licenses_flutter b/ci/licenses_golden/licenses_flutter
index 111447feb7a90..d3d41033a1040 100644
--- a/ci/licenses_golden/licenses_flutter
+++ b/ci/licenses_golden/licenses_flutter
@@ -42761,7 +42761,6 @@ ORIGIN: ../../../flutter/lib/ui/window/pointer_data_packet_converter.cc + ../../
 ORIGIN: ../../../flutter/lib/ui/window/pointer_data_packet_converter.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/ui/window/viewport_metrics.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/ui/window/viewport_metrics.h + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/lib/web_ui/flutter_js/src/base_uri.js + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/flutter_js/src/browser_environment.js + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/flutter_js/src/canvaskit_loader.js + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/flutter_js/src/entrypoint_loader.js + ../../../flutter/LICENSE
@@ -42771,6 +42770,7 @@ ORIGIN: ../../../flutter/lib/web_ui/flutter_js/src/loader.js + ../../../flutter/
 ORIGIN: ../../../flutter/lib/web_ui/flutter_js/src/service_worker_loader.js + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/flutter_js/src/skwasm_loader.js + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/flutter_js/src/trusted_types.js + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/lib/web_ui/flutter_js/src/utils.js + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/annotations.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/canvas.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/channel_buffers.dart + ../../../flutter/LICENSE
@@ -45632,7 +45632,6 @@ FILE: ../../../flutter/lib/ui/window/pointer_data_packet_converter.cc
 FILE: ../../../flutter/lib/ui/window/pointer_data_packet_converter.h
 FILE: ../../../flutter/lib/ui/window/viewport_metrics.cc
 FILE: ../../../flutter/lib/ui/window/viewport_metrics.h
-FILE: ../../../flutter/lib/web_ui/flutter_js/src/base_uri.js
 FILE: ../../../flutter/lib/web_ui/flutter_js/src/browser_environment.js
 FILE: ../../../flutter/lib/web_ui/flutter_js/src/canvaskit_loader.js
 FILE: ../../../flutter/lib/web_ui/flutter_js/src/entrypoint_loader.js
@@ -45643,6 +45642,7 @@ FILE: ../../../flutter/lib/web_ui/flutter_js/src/service_worker_loader.js
 FILE: ../../../flutter/lib/web_ui/flutter_js/src/skwasm_loader.js
 FILE: ../../../flutter/lib/web_ui/flutter_js/src/trusted_types.js
 FILE: ../../../flutter/lib/web_ui/flutter_js/src/types.d.ts
+FILE: ../../../flutter/lib/web_ui/flutter_js/src/utils.js
 FILE: ../../../flutter/lib/web_ui/lib/annotations.dart
 FILE: ../../../flutter/lib/web_ui/lib/canvas.dart
 FILE: ../../../flutter/lib/web_ui/lib/channel_buffers.dart

From a6b80366e14172fb452fac418895e1added25eb6 Mon Sep 17 00:00:00 2001
From: Jackson Gardner <jacksongardner@google.com>
Date: Fri, 7 Jun 2024 10:35:03 -0700
Subject: [PATCH 4/4] David's review comments.

---
 lib/web_ui/flutter_js/src/loader.js |  4 +--
 lib/web_ui/flutter_js/src/utils.js  | 40 ++++++++++++++++++-----------
 2 files changed, 27 insertions(+), 17 deletions(-)

diff --git a/lib/web_ui/flutter_js/src/loader.js b/lib/web_ui/flutter_js/src/loader.js
index 1d4e3c6c37322..c24f3765a57ca 100644
--- a/lib/web_ui/flutter_js/src/loader.js
+++ b/lib/web_ui/flutter_js/src/loader.js
@@ -8,7 +8,7 @@ import { FlutterServiceWorkerLoader } from './service_worker_loader.js';
 import { FlutterTrustedTypesPolicy } from './trusted_types.js';
 import { loadCanvasKit } from './canvaskit_loader.js';
 import { loadSkwasm } from './skwasm_loader.js';
-import { getDefaultCanvaskitBaseUrl } from './utils.js';
+import { getCanvaskitBaseUrl } from './utils.js';
 
 /**
  * The public interface of _flutter.loader. Exposes two methods:
@@ -127,7 +127,7 @@ export class FlutterLoader {
       });
     }
 
-    const canvasKitBaseUrl = getDefaultCanvaskitBaseUrl(config, buildConfig);
+    const canvasKitBaseUrl = getCanvaskitBaseUrl(config, buildConfig);
     if (build.renderer === "canvaskit") {
       deps.canvasKit = loadCanvasKit(deps, config, browserEnvironment, canvasKitBaseUrl);
     } else if (build.renderer === "skwasm") {
diff --git a/lib/web_ui/flutter_js/src/utils.js b/lib/web_ui/flutter_js/src/utils.js
index 8f52db6217855..ecd4c2c21b937 100644
--- a/lib/web_ui/flutter_js/src/utils.js
+++ b/lib/web_ui/flutter_js/src/utils.js
@@ -10,25 +10,35 @@ function getBaseURI() {
 }
 
 export function joinPathSegments(...segments) {
-  return segments.map((segment, i) => {
-    if (!segment) {
-      return "";
-    }
-
+  return segments.filter((segment) => !!segment).map((segment, i) => {
     if (i === 0) {
-      return stripRightSlash(segment);
+      return stripRightSlashes(segment);
     } else {
-      return stripLeftSlash(stripRightSlash(segment));
+      return stripLeftSlashes(stripRightSlashes(segment));
     }
-  }).filter(x => x.length).join('/')
+  }).filter(x => x.length).join("/")
 }
 
-function stripLeftSlash(s) {
-  return s.startsWith('/') ? s.substring(1) : s;
+function stripLeftSlashes(s) {
+  let i = 0;
+  while (i < s.length) {
+    if (s.charCodeAt(i) !== "/") {
+      break;
+    }
+    i++;
+  }
+  return s.substring(i);
 }
 
-function stripRightSlash(s) {
-  return s.endsWith('/') ? s.substring(0, s.length - 1) : s;
+function stripRightSlashes(s) {
+  let i = s.length;
+  while (i > 0) {
+    if (s.charCodeAt(i - 1) !== "/") {
+      break;
+    }
+    i--;
+  }
+  return s.substring(0, i);
 }
 
 /**
@@ -37,12 +47,12 @@ function stripRightSlash(s) {
  * @param {import("./types").FlutterConfiguration} config
  * @param {import("./types").BuildConfig} buildConfig
  */
-export function getDefaultCanvaskitBaseUrl(config, buildConfig) {
+export function getCanvaskitBaseUrl(config, buildConfig) {
   if (config.canvasKitBaseUrl) {
     return config.canvasKitBaseUrl;
   }
   if (buildConfig.engineRevision && !buildConfig.useLocalCanvasKit) {
-    return joinPathSegments('https://www.gstatic.com/flutter-canvaskit', buildConfig.engineRevision);
+    return joinPathSegments("https://www.gstatic.com/flutter-canvaskit", buildConfig.engineRevision);
   }
-  return '/canvaskit';
+  return "/canvaskit";
 }