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