anchor.diff:  
   class ColorFilter extends OneChildRenderObjectWrapper {
     ColorFilter({ Key key, this.color, this.transferMode, Widget child })
  -    : super(key: key, child: child);
  +    : super(key: key, child: child) {
  +    assert(color != null);
  +    assert(transferMode != null);
  +  }
   
     final Color color;
     final sky.TransferMode transferMode;
  
positive.code:     this.decoration,
      this.position: BoxDecorationPosition.background,
      Widget child
    }) : super(key: key, child: child);
  
    final BoxDecoration decoration;
    final BoxDecorationPosition position;
  
negative.code:     this.scrollDirection: ScrollDirection.vertical,
      this.scrollOffset: Offset.zero,
      Widget child
    }) : super(key: key, child: child) {
      assert(scrollDirection != null);
      assert(scrollOffset != null);
    }
  
    final ScrollDirection scrollDirection;
    final Offset scrollOffset;
  