[flutter_tools] fix frontend server generated entrypoint (#50036)

packages/flutter_tools/lib/src/build_runner/resident_web_runner.dart

@@ -515,7 +515,7 @@ class _ExperimentalResidentWebRunner extends ResidentWebRunner {
           'import "$generatedImport";',
         'Future<void> main() async {',
         if (hasWebPlugins)
-          '  registerPlugins(webPluginRegistry);'
+          '  registerPlugins(webPluginRegistry);',
         '  await ui.webOnlyInitializePlatform();',
         '  entrypoint.main();',
         '}',

packages/flutter_tools/test/general.shard/resident_web_runner_test.dart

@@ -414,6 +414,7 @@ void main() {
   test('Can hot restart after attaching - experimental', () => testbed.run(() async {
     _setupMocks();
     launchChromeInstance(mockChrome);
+    String entrypointFileName;
     when(mockWebDevFS.update(
       mainPath: anyNamed('mainPath'),
       target: anyNamed('target'),
@@ -428,6 +429,7 @@ void main() {
       pathToReload: anyNamed('pathToReload'),
       invalidatedFiles: anyNamed('invalidatedFiles'),
     )).thenAnswer((Invocation invocation) async {
+      entrypointFileName = invocation.namedArguments[#mainPath] as String;
       return UpdateFSReport(success: true)
         ..invalidatedModules = <String>['example'];
     });
@@ -438,6 +440,12 @@ void main() {
     await connectionInfoCompleter.future;
     final OperationResult result = await residentWebRunner.restart(fullRestart: true);
 
+    // Ensure that generated entrypoint is generated correctly.
+    expect(entrypointFileName, isNotNull);
+    expect(globals.fs.file(entrypointFileName).readAsStringSync(), contains(
+      'await ui.webOnlyInitializePlatform();'
+    ));
+
     expect(testLogger.statusText, contains('Restarted application in'));
     expect(result.code, 0);
     verify(mockResidentCompiler.accept()).called(2);