anchor.diff:  }
   
   class Text extends Component {
  -  Text(this.data, { Key key, TextStyle this.style }) : super(key: key);
  +  Text(this.data, { Key key, TextStyle this.style }) : super(key: key) {
  +    assert(data != null);
  +  }
   
     final String data;
     final TextStyle style;
  
positive.code: 
  class SizeObserver extends OneChildRenderObjectWrapper {
    SizeObserver({ Key key, this.callback, Widget child })
      : super(key: key, child: child);
  
    final SizeChangedCallback callback;
  
  
negative.code: 
    RenderStack createNode() => new RenderStack();
    RenderStack get renderObject => super.renderObject;
  }
  
  class Positioned extends ParentDataNode {
  