anchor.diff:    Block(List<Widget> children, { String key })
       : super(key: key, children: children);
   
  +  RenderBlock createNode() => new RenderBlock();
     RenderBlock get root => super.root;
  -  RenderBlock createNode() => new RenderBlock();
   }
   
   class Stack extends MultiChildRenderObjectWrapper {
     Stack(List<Widget> children, { String key })
       : super(key: key, children: children);
   
  +  RenderStack createNode() => new RenderStack();
     RenderStack get root => super.root;
  -  RenderStack createNode() => new RenderStack();
   }
   
   class Positioned extends ParentDataNode {
  
positive.code:   void remove() {
      RenderObjectWrapper ancestor = findAncestorRenderObjectWrapper();
      assert(ancestor is RenderObjectWrapper);
      ancestor.detachChildRoot(this);
      super.remove();
    }
  
  }
  
negative.code:   final dynamic elements;
  
    InlineBase _toInline(dynamic element) {
      if (element is String)
        return new InlineText(element);
      if (element is Iterable && element.first is TextStyle)
        return new InlineStyle(element.first, element.skip(1).map(_toInline).toList());
      throw new ArgumentError("invalid elements");
    }
  
  