anchor.diff:        scrollTo(1.0 - performance.progress);
     }
   
  -  int get itemCount => _itemIndices.length;
  -
     void dispatchOnScroll() {
       if (_selection == null || _selection.valueIsChanging)
         return;
  
positive.code: class _TabBarViewState<T> extends PageableListState<TabBarView<T>> implements TabBarSelectionPerformanceListener {
  
    TabBarSelectionState _selection;
    List<int> _itemIndices = [0, 1];
    AnimationDirection _scrollDirection = AnimationDirection.forward;
  
    int get _tabCount => config.items.length;
  
negative.code:   }) : items = items, itemBuilder = itemBuilder, super(
      key: key,
      scrollDirection: ScrollDirection.horizontal,
      children: items.map((T item) => itemBuilder(item)).toList(),
      itemsWrap: false
    ) {
      assert(items != null);
  