Rename all the "2" classes related to scrolling to remove the "2". Now that the
old scrolling code is gone, we don't need to use the suffix.

There appears to be an issue with children before the center widget.
I've filled a bug about that issue and will follow up in a later patch.

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.)