refactor: redesign flutter widget elements support.

[andycall]

2. Flutter Widget Inspector now can inspect all custom elements.
3. Perfect lifecycle match between Flutter Widgets and WebF Elements.

4. Optimize WidgetElement property and method query performance.

The performance problems

This patch will ignore to pass properties when it's not defined in WidgetElements.

let startTime = Date.now();
let sampleElement = document.createElement('flutter-widget-element');
let obj =  { complex: true }; // This is a complex data object.
for(let i = 0; i < 10000; i ++) {
  sampleElement._cachedData = obj; // The "_cachedData" is not defined on Dart side. But it still takes time and sends to Dart. 
}
console.log(Date.now() - startTime);