Don't eagerly call runMain when --start-paused is provided to web application (#42144)

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

@@ -259,7 +259,17 @@ class ResidentWebRunner extends ResidentRunner {
       unawaited(_vmService.registerService('reloadSources', 'FlutterTools'));
       websocketUri = Uri.parse(_connectionResult.debugConnection.uri);
       // Always run main after connecting because start paused doesn't work yet.
-      _connectionResult.appConnection.runMain();
+      if (!debuggingOptions.startPaused || !supportsServiceProtocol) {
+        _connectionResult.appConnection.runMain();
+      } else {
+        StreamSubscription<void> resumeSub;
+        resumeSub = _connectionResult.debugConnection.vmService.onDebugEvent.listen((vmservice.Event event) {
+          if (event.type == vmservice.EventKind.kResume) {
+            _connectionResult.appConnection.runMain();
+            resumeSub.cancel();
+          }
+        });
+      }
     }
     if (websocketUri != null) {
       printStatus('Debug service listening on $websocketUri');

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

@@ -83,6 +83,9 @@ void main() {
     when(mockVmService.onStdoutEvent).thenAnswer((Invocation _) {
       return const Stream<Event>.empty();
     });
+    when(mockVmService.onDebugEvent).thenAnswer((Invocation _) {
+      return const Stream<Event>.empty();
+    });
     when(mockDebugConnection.uri).thenReturn('ws://127.0.0.1/abcd/');
   }
 
@@ -174,6 +177,27 @@ void main() {
     expect(bufferLogger.statusText, contains('THIS MESSAGE IS IMPORTANT'));
   }));
 
+  test('Does not run main with --start-paused', () => testbed.run(() async {
+    residentWebRunner = ResidentWebRunner(
+      mockWebDevice,
+      flutterProject: FlutterProject.current(),
+      debuggingOptions: DebuggingOptions.enabled(BuildInfo.debug, startPaused: true),
+      ipv6: true,
+    );
+    _setupMocks();
+    final Completer<DebugConnectionInfo> connectionInfoCompleter = Completer<DebugConnectionInfo>();
+    final StreamController<Event> controller = StreamController<Event>.broadcast();
+    when(mockVmService.onStdoutEvent).thenAnswer((Invocation _) {
+      return controller.stream;
+    });
+    unawaited(residentWebRunner.run(
+      connectionInfoCompleter: connectionInfoCompleter,
+    ));
+    await connectionInfoCompleter.future;
+
+    verifyNever(mockAppConnection.runMain());
+  }));
+
   test('Can hot reload after attaching', () => testbed.run(() async {
     _setupMocks();
     final BufferLogger bufferLogger = logger;