Add children setter

It would be shorter to have a children setter instead the addChild Method, for example instead of:
```
void main() {
  new View()
    ..addChild(new TextView("Hello World!")
    ..on.click.listen((event) {
      (event.target as TextView).text = "Welcome to Rikulo.";
      event.target.requestLayout();
    }), new TextView("Hello World!"))
    ..addChild(new View()
      ..layout.type = "linear" //arrange the layout of child views linearly
      ..addChild(new TextView("Name")) //a label
      ..addChild(new TextBox()))
    ..addToDocument(); 

}
```
would be shorter to have:
```
new View()
    ..children = [
      new TextView("Hello World!")
        ..on.click.listen((event) {
          (event.target as TextView).text = "Welcome to Rikulo.";
          event.target.requestLayout();
      }),
      new View()
        ..layout.type = "linear" //arrange the layout of child views linearly
        ..children = [
          new TextView("Name"), //a label
          new TextBox()
      ]
   ]
   ..addToDocument(); 
```
I think that it could be solved adding this code to the View class: