anchor.diff:    }
   }
   
  -class TabBarView<T> extends ScrollableList<T> {
  +class TabBarView<T> extends PageableList<T> {
     TabBarView({
       Key key,
       this.selection,
       List<T> items,
  -    ItemBuilder<T> itemBuilder,
  -    double itemExtent
  +    ItemBuilder<T> itemBuilder
     }) : super(
       key: key,
       scrollDirection: ScrollDirection.horizontal,
       items: items,
       itemBuilder: itemBuilder,
  -    itemExtent: itemExtent,
       itemsWrap: false
     ) {
       assert(selection != null);
  
positive.code: const double _kMaxTabWidth = 264.0;
  const EdgeDims _kTabLabelPadding = const EdgeDims.symmetric(horizontal: 12.0);
  const double _kTabBarScrollDrag = 0.025;
  const Duration _kTabBarScroll = const Duration(milliseconds: 200);
  
  class _TabBarParentData extends ContainerBoxParentDataMixin<RenderBox> { }
  
  
negative.code: 
    void _handleProgressChange() {
      if (_scrollDirection == AnimationDirection.forward)
        scrollTo(_performance.progress);
      else
        scrollTo(1.0 - _performance.progress);
    }
  
    int get itemCount => _itemIndices.length;
  