anchor.diff:      this.scrollDirection: ScrollDirection.vertical,
       this.scrollOffset: Offset.zero,
       Widget child
  -  }) : super(key: key, child: child);
  +  }) : super(key: key, child: child) {
  +    assert(scrollDirection != null);
  +    assert(scrollOffset != null);
  +  }
   
     final ScrollDirection scrollDirection;
     final Offset scrollOffset;
  
positive.code: }
  
  class Text extends Component {
    Text(this.data, { Key key, TextStyle this.style }) : super(key: key);
  
    final String data;
    final TextStyle style;
  
negative.code: 
  class SizeObserver extends OneChildRenderObjectWrapper {
    SizeObserver({ Key key, this.callback, Widget child })
      : super(key: key, child: child) {
      assert(callback != null);
    }
  
    final SizeChangedCallback callback;
  
  