anchor.diff:      Map<int, Widget> builtChildren = new Map<int, Widget>();
   
       final List<double> offsets = layoutState._childOffsets;
  -    final Map<_Key, Widget> childrenByKey = layoutState._childrenByKey;
  +    final Map<_Key, Widget> childrenByKey = _childrenByKey;
       double extent;
       if (direction == ScrollDirection.vertical) {
         extent = constraints.maxHeight;
  
positive.code:       }
      }
  
      layoutState._childrenByKey = newChildren;
      layoutState._firstVisibleChildIndex = startIndex;
      layoutState._visibleChildCount = newChildren.length;
    }
  
negative.code:       return false;
      });
      return null;
    }
  
    int _findIndexForOffsetBeforeOrAt(double offset) {
  