anchor.diff:    }
   
     void layout(BoxConstraints constraints) {
  +    if (!_dirty)
  +      return;
       _dirty = false;
   
       Map<_Key, Widget> newChildren = new Map<_Key, Widget>();
  
positive.code: 
    void syncRenderObject(BlockViewport old) {
      super.syncRenderObject(old);
      if (_dirty)
        root.markNeedsLayout();
    }
  
    Widget _getWidget(int index, BoxConstraints innerConstraints) {
  
negative.code:     }
  
      _childrenByKey = newChildren;
      _currentStartIndex = startIndex;
      _currentChildCount = _childrenByKey.length;
    }
  
  }
  