iframe中的javascript如何与dart通信

Question

ui.platformViewRegistry.registerViewFactory(
  'subpage',
        (int viewId) => IFrameElement()
      ..src = 'https://127.0.0.1/pages/index.html'
      ..style.border = 'none');
return HtmlElementView(viewType: 'subpage');

index.html中的javascript如何与dart通信？