anchor.diff:  
   abstract class MultiChildRenderObjectWrapper extends RenderObjectWrapper {
   
  -  // In MultiChildRenderObjectWrapper subclasses, slots are RenderObject nodes
  -  // to use as the "insert before" sibling in ContainerRenderObjectMixin.add() calls
  +  // In MultiChildRenderObjectWrapper subclasses, slots are the Widget
  +  // nodes whose RenderObjects are to be used as the "insert before"
  +  // sibling in ContainerRenderObjectMixin.add() calls
   
     MultiChildRenderObjectWrapper({ Key key, List<Widget> children })
       : this.children = children == null ? const [] : children,
  
positive.code:     assert(renderObject is RenderBox);
      return (renderObject as RenderBox).localToGlobal(point);
    }
  }
  
  
  
negative.code:       if (_ancestor is RenderObjectWrapper)
          _ancestor.insertChildRenderObject(this, slot);
      } else {
        assert(_canSync(this, old));
        _renderObject = old.renderObject;
        _ancestor = old._ancestor;
        assert(_renderObject != null);
  