This prepares us for a CustomScrollView that takes slivers.

This patch introduces ScrollView, which is a convenience widget for using a
SliverBlock. This patch also switches a number of tests from Block to
ScrollView. Once we support more features of block (e.g., padding and
shrinkwrapping), we'll be able to move over more clients.

Also, make the exception handling for global key listeners slightly more
robust.

Our previous approach to detecting when we needed to remove semantics nodes
didn't account for reparenting.