[web] Fix paths fetched by flutter.js (#118684)

packages/flutter_tools/lib/src/web/file_generators/flutter_js.dart

@@ -20,6 +20,21 @@ _flutter.loader = null;
 
 (function () {
   "use strict";
+
+  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 : `${uri}/`;
+  }
+
   /**
    * Wraps `promise` in a timeout of the given `duration` in ms.
    *
@@ -120,8 +135,7 @@ _flutter.loader = null;
       }
       const {
         serviceWorkerVersion,
-        serviceWorkerUrl = "flutter_service_worker.js?v=" +
-          serviceWorkerVersion,
+        serviceWorkerUrl = `${baseUri}flutter_service_worker.js?v=${serviceWorkerVersion}`,
         timeoutMillis = 4000,
       } = settings;
 
@@ -239,7 +253,7 @@ _flutter.loader = null;
      * Returns undefined when an `onEntrypointLoaded` callback is supplied in `options`.
      */
     async loadEntrypoint(options) {
-      const { entrypointUrl = "main.dart.js", onEntrypointLoaded } =
+      const { entrypointUrl = `${baseUri}main.dart.js`, onEntrypointLoaded } =
         options || {};
 
       return this._loadEntrypoint(entrypointUrl, onEntrypointLoaded);

packages/flutter_tools/test/general.shard/build_system/targets/web_test.dart

@@ -852,6 +852,18 @@ void main() {
       contains('"main.dart.js"'));
   }));
 
+  test('flutter.js sanity checks', () {
+    final String flutterJsContents = flutter_js.generateFlutterJsFile();
+    expect(flutterJsContents, contains('"use strict";'));
+    expect(flutterJsContents, contains('main.dart.js'));
+    expect(flutterJsContents, contains('flutter_service_worker.js?v='));
+    expect(flutterJsContents, contains('document.createElement("script")'));
+    expect(flutterJsContents, contains('"application/javascript"'));
+    expect(flutterJsContents, contains('const baseUri = '));
+    expect(flutterJsContents, contains('document.querySelector("base")'));
+    expect(flutterJsContents, contains('.getAttribute("href")'));
+  });
+
   test('flutter.js is not dynamically generated', () => testbed.run(() async {
     globals.fs.file('bin/cache/flutter_web_sdk/canvaskit/foo')
       ..createSync(recursive: true)