Changed the pageable_list.dart example: tapping on the toolbar changes
the scroll direction. This exposed some problems:

- Scrollable.syncFields() didn't update scrollDirection
- Viewport updated its RenderObject fields in the wrong order
- FixedHeightScrollable scrollDirection changes didn't update the scrollBehavior

There may be similar problems with VariableHeightList and ScrollableViewport.
I will fix those in a separate CL.