dart与js的互操作（一）

https://dart.dev/interop/js-interop/past-js-interop

@js.JSExport()
class _MyAppState extends State<MyApp> {
  final _streamController = StreamController<void>.broadcast();
  DemoScreen _currentDemoScreen = DemoScreen.counter;
  int _counterScreenCount = 0;

  @override
  void initState() {
    super.initState();
    final export = js.createJSInteropWrapper(this);
    js.globalContext['_appState'] = export;
    js.globalContext.callMethod('_stateSet'.toJS); // 调用js方法，将Dart实例化
  }