anchor.diff:      _boundedBehavior ??= new BoundedBehavior();
       return _boundedBehavior;
     }
  -
   
     void _initSelection(TabBarSelectionState<T> selection) {
       _selection = selection;
  
positive.code:     TabBarSelectionState<T> newSelection = TabBarSelection.of(context);
      if (_selection != newSelection)
        _initSelection(newSelection);
      return super.buildContent(context);
    }
  }
  
negative.code: class _TabBarViewState<T> extends PageableListState<TabBarView<T>> implements TabBarSelectionPerformanceListener {
  
    TabBarSelectionState _selection;
    List<Widget> _items;
    AnimationDirection _scrollDirection = AnimationDirection.forward;
  
    int get _tabCount => config.items.length;
  