anchor.diff:      }
   
       _childrenByKey = newChildren;
  +    _currentStartIndex = startIndex;
  +    _currentChildCount = _childrenByKey.length;
     }
   
   }
  
positive.code:   // end of the last one. If there's no children, then the only offset
    // is 0.0.
    List<double> _offsets = <double>[0.0];
  
    int _findIndexForOffsetBeforeOrAt(double offset) {
      int left = 0;
  
negative.code:   }
  
    void layout(BoxConstraints constraints) {
      if (!_dirty)
        return;
      _dirty = false;
  
      Map<_Key, Widget> newChildren = new Map<_Key, Widget>();
  