anchor.diff:  class PageableList<T> extends ScrollableList<T> {
     PageableList({
       Key key,
  +    double initialScrollOffset,
       ScrollDirection scrollDirection: ScrollDirection.horizontal,
       List<T> items,
       ItemBuilder<T> itemBuilder,
  
positive.code: class ScrollableMixedWidgetList extends Scrollable {
    ScrollableMixedWidgetList({
      Key key,
      this.builder,
      this.token,
      this.layoutState
    }) : super(key: key);
  
    IndexedBuilder builder;
    Object token;
  
negative.code: 
    Widget build() {
      return new ScrollableViewport(
        initialScrollOffset: initialScrollOffset,
        scrollDirection: scrollDirection,
        child: new BlockBody(children, direction: _direction)
      );
  