This patch makes Scrollable2.ensureVisible with SingleChildScrollView. A future
patch will extend the implementation to work with slivers. (Although the patch
does include some of the infrastructure for that part of the implementation as
well.)

This property is wired up to anything and it isn't used. We'll need to find a
better way for clients to control the scroll offset.

This structure makes it easier to customize the viewport used in a Scrollable.
Now Scrollable2 doesn't commit to using slivers.

This widget is a replacement for ScrollableViewport that uses the new
Scrollable2 machinery. The widget is not based on Slivers but does use the new
scroll behavior classes.