anchor.diff:            assert(widget != null);
             assert(widget.key != null);
             _Key key = new _Key.fromWidget(widget);
  -          Widget oldWidget = layoutState._childrenByKey[key];
  +          Widget oldWidget = _childrenByKey[key];
             assert(oldWidget != null);
             assert(oldWidget.renderObject.parent == renderObject);
             widget = syncChild(widget, oldWidget, renderObject.childAfter(oldWidget.renderObject));
             assert(widget != null);
  -          layoutState._childrenByKey[key] = widget;
  +          _childrenByKey[key] = widget;
           }
         }
       }
  
positive.code:     _readOnlyChildOffsets = new UnmodifiableListView<double>(_childOffsets);
    }
  
    Map<_Key, Widget> _childrenByKey = new Map<_Key, Widget>();
    bool _dirty = true;
  
    int _firstVisibleChildIndex;
  
negative.code:     Map<int, Widget> builtChildren = new Map<int, Widget>();
  
      final List<double> offsets = layoutState._childOffsets;
      final Map<_Key, Widget> childrenByKey = _childrenByKey;
      double extent;
      if (direction == ScrollDirection.vertical) {
        extent = constraints.maxHeight;
  