Feat/support flutter widget

[answershuto]

支持 Flutter Widget 与 Element 混合渲染，互相嵌套。

Example:

let container = document.createElement('div');
container.style.background = 'red';
container.style.border = '1px solid #000';
container.appendChild(document.createTextNode('helloworld'));

let flutterText = document.createElement('flutter-list-view');
flutterText.appendChild(container);
flutterText.style.border = '1px solid #000';
flutterText.style.display = 'inline-block';
flutterText.style.width = '100px';
flutterText.style.height = '500px';
flutterText.onclick = () =>  console.log(1);

document.body.appendChild(flutterText)

void main() {
  Kraken.defineCustomElement('flutter-list-view', (targetId, nativeEventTarget, elementManager) => FlutterListView(targetId, nativeEventTarget, elementManager));
  runApp(MyApp());
}

class FlutterListView extends WidgetElement {
  FlutterListView(int targetId, Pointer<NativeEventTarget> nativeEventTarget, ElementManager elementManager) : super(targetId, nativeEventTarget, elementManager);

  @override
  Widget build(BuildContext context, Map<String, dynamic> properties, List<Widget> children) {
    return ListView(
      padding: const EdgeInsets.all(8),
      children: <Widget>[
        Container(
          height: 50,
          child: children.isNotEmpty ? children[0] : null,
        ),
        Container(
          height: 50,
          color: Colors.amber[500],
          child: const Center(child: Text('Entry B')),
        ),
        Container(
          height: 50,
          color: Colors.amber[100],
          child: const Center(child: Text('Entry C')),
        ),
      ],
    );
  }
}

[haikyuu]

This isn't related to HTML custom elements right?

[yuanyan]

haikyuu

Not web components，web components will support in future.

[andycall]

每次刷新，整个列表都要重新 rebuild，性能瓶颈会很明显。需要优化一下，缓存前面已经构建好的 widget

[answershuto]

这里子节点会调用 WidgetElement 上的 build 方法，该方法也许会依赖父节点的信息做一些转换，重新 build 把优化的权利交给开发者，这里不缓存防止需要重新 build 的时候不执行。KrakenElementToWidgetAdaptor 我加上缓存。

[andycall]

一些优化需要调整。貌似 main 上 widget.dart 没有同步过来，需要手动 check 一下