implement debugTogglePlatform for the web (#42951)

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

@@ -384,6 +384,34 @@ class ResidentWebRunner extends ResidentRunner {
     }
   }
 
+  @override
+  Future<void> debugTogglePlatform() async {
+    try {
+      final vmservice.Response response = await _vmService.callServiceExtension(
+          'ext.flutter.platformOverride');
+      final String currentPlatform = response.json['value'];
+      String nextPlatform;
+      switch (currentPlatform) {
+        case 'android':
+          nextPlatform = 'iOS';
+          break;
+        case 'iOS':
+          nextPlatform = 'android';
+          break;
+      }
+      if (nextPlatform == null) {
+        return;
+      }
+      await _vmService.callServiceExtension(
+        'ext.flutter.platformOverride', args: <String, Object>{
+          'value': nextPlatform,
+        });
+      printStatus('Switched operating system to $nextPlatform');
+    } on vmservice.RPCError {
+      return;
+    }
+  }
+
   @override
   Future<void> debugDumpSemanticsTreeInInverseHitTestOrder() async {
     try {

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

@@ -435,6 +435,26 @@ void main() {
         args: <String, Object>{'enabled': true})).called(1);
   }));
 
+  test('debugTogglePlatform', () => testbed.run(() async {
+    _setupMocks();
+    final BufferLogger bufferLogger = logger;
+    final Completer<DebugConnectionInfo> connectionInfoCompleter = Completer<DebugConnectionInfo>();
+    unawaited(residentWebRunner.run(
+      connectionInfoCompleter: connectionInfoCompleter,
+    ));
+    await connectionInfoCompleter.future;
+    when(mockVmService.callServiceExtension('ext.flutter.platformOverride'))
+      .thenAnswer((Invocation _) async {
+        return Response.parse(<String, Object>{'value': 'iOS'});
+    });
+
+    await residentWebRunner.debugTogglePlatform();
+
+    expect(bufferLogger.statusText, contains('Switched operating system to android'));
+    verify(mockVmService.callServiceExtension('ext.flutter.platformOverride',
+        args: <String, Object>{'value': 'android'})).called(1);
+  }));
+
   test('cleanup of resources is safe to call multiple times', () => testbed.run(() async {
     _setupMocks();
     bool debugClosed = false;