anchor.diff:    BlockBody(List<Widget> children, {
       Key key,
       this.direction: BlockDirection.vertical
  -  }) : super(key: key, children: children);
  +  }) : super(key: key, children: children) {
  +    assert(direction != null);
  +  }
   
     final BlockDirection direction;
   
  
positive.code: 
  class Opacity extends OneChildRenderObjectWrapper {
    Opacity({ Key key, this.opacity, Widget child })
      : super(key: key, child: child);
  
    final double opacity;
  
  
negative.code: 
  class Padding extends OneChildRenderObjectWrapper {
    Padding({ Key key, this.padding, Widget child })
      : super(key: key, child: child) {
      assert(padding != null);
    }
  
    final EdgeDims padding;
  
  