anchor.diff:  class PageableList<T> extends ScrollableList<T> {
     PageableList({
       Key key,
  -    ScrollDirection scrollDirection,
  +    ScrollDirection scrollDirection: ScrollDirection.horizontal,
       List<T> items,
       ItemBuilder<T> itemBuilder,
       double itemExtent,
  
positive.code: // about offscreen widgets consuming resources.
  abstract class FixedHeightScrollable extends Scrollable {
  
    FixedHeightScrollable({ Key key, ScrollDirection scrollDirection, this.itemExtent, this.padding })
        : super(key: key, scrollDirection: scrollDirection) {
      assert(itemExtent != null);
    }
  
  
negative.code: class ScrollableList<T> extends FixedHeightScrollable {
    ScrollableList({
      Key key,
      ScrollDirection scrollDirection: ScrollDirection.vertical,
      this.items,
      this.itemBuilder,
      double itemExtent,
  