anchor.diff:        return false;
       });
       return null;
  -  }
  -
  -  void didMount() {
  -    renderObject.callback = layout;
  -    renderObject.totalExtentCallback = _noIntrinsicDimensions;
  -    renderObject.maxCrossAxisDimensionCallback = _noIntrinsicDimensions;
  -    renderObject.minCrossAxisDimensionCallback = _noIntrinsicDimensions;
  -    super.didMount();
  -  }
  -
  -  void didUnmount() {
  -    renderObject.callback = null;
  -    renderObject.totalExtentCallback = null;
  -    renderObject.maxCrossAxisDimensionCallback = null;
  -    renderObject.minCrossAxisDimensionCallback = null;
  -    super.didUnmount();
     }
   
     int _findIndexForOffsetBeforeOrAt(double offset) {
  
positive.code:       }
      }
  
      layoutState._childrenByKey = newChildren;
      layoutState._firstVisibleChildIndex = startIndex;
      layoutState._visibleChildCount = newChildren.length;
    }
  
negative.code:       return null;
      assert(widget.key != null); // items in lists must have keys
      final _Key key = new _Key.fromWidget(widget);
      Widget oldWidget = _childrenByKey[key];
      widget = syncChild(widget, oldWidget, _omit);
      if (index >= offsets.length - 1) {
        assert(index == offsets.length - 1);
  