anchor.diff:  ///
   /// Subclasses typically override [buildContent] to build viewports.
   abstract class PageableState<T extends Pageable> extends ScrollableState<T> {
  -  int get _itemCount => config._itemCount;
  +  int get _itemCount => config.itemCount;
     int _previousItemCount;
   
  -  double get _pixelsPerScrollUnit {
  +  /// Convert from the item based scroll units to logical pixels.
  +  double get pixelsPerScrollUnit {
       final RenderBox box = context.findRenderObject();
       if (box == null || !box.hasSize)
         return 0.0;
  
positive.code:   final Iterable<Widget> children;
  
    @override
    int get _itemCount => children?.length ?? 0;
  
    @override
    PageableListState<PageableList> createState() => new PageableListState<PageableList>();
  
negative.code: 
    @override
    double pixelOffsetToScrollOffset(double pixelOffset) {
      final double unit = pixelsPerScrollUnit;
      return super.pixelOffsetToScrollOffset(unit == 0.0 ? 0.0 : pixelOffset / unit);
    }
  
    @override
    double scrollOffsetToPixelOffset(double scrollOffset) {
      return super.scrollOffsetToPixelOffset(scrollOffset * pixelsPerScrollUnit);
    }
  
    int _scrollOffsetToPageIndex(double scrollOffset) {
  