TransitionRoute completer is now optional.

Factored OverlayRoute out of the modal and persistent bottom sheet clases, since the bottom sheet classes need to drive the performance.

Added a bottom sheet to the stocks demo: long-press on a stock shows a modal bottom sheet.

Made AnimatedModalBarrier public.

Previously, we passed widgets up the hierarchy to display them in the overlay,
but that breaks the change propagation logic because those widgets won't get
rebuilt.  Now we pass WidgetBuilders instead, which can be rebuilt when the
overlay rebuilds.

Fixes #1913

This patch introduces the notion of a ModalRoute that puts up a modal barrier
and makes the route invisible to hit testing when its animating out. This patch
also uses this mechanism in a number of places (including PageRoute). There are
still a few more cases to convert, but that's work for a future patch.

Fixes #1684