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:     Map<int, Widget> builtChildren = new Map<int, Widget>();
  
      final List<double> offsets = layoutState._childOffsets;
      final Map<_Key, Widget> childrenByKey = layoutState._childrenByKey;
      double extent;
      if (direction == ScrollDirection.vertical) {
        extent = constraints.maxHeight;
  
negative.code:     _readOnlyChildOffsets = new UnmodifiableListView<double>(_childOffsets);
    }
  
    bool _dirty = true;
  
    int _firstVisibleChildIndex;
  