The pointer router was using an iteration pattern that always delivers
handleEvent calls even if you remove a route during the iteration.
That's awkward to program against and causes trouble for the double-tap
gesture.

This patch switches PointerRouter to using a re-entrant iteration
pattern that supports removing routes (but not adding routes) during the
iteration.

Instead of PointerInputEvent having a "type" field, we now have a
different class for each pointer type.

This has ripple effects throughout the system.

I also did code cleanup in affected files while I was there.

This package contains WidgetTester, which is very useful when writing tests for
widgets.

This patch is part of a sequence of patches towards fewer top-level libraries.
In this patch, the gesture libraries are combined into one gestures.dart
library.

These files really belong on other libraries.

If there are no other gestures in the arena, we should kick off the scroll
gesture right away. This change pulled a refactoring of how we dispatch events
to Widgets. Now we dispatch events to Widgets interleaved with their associated
RenderObjects. (Previously we dispatched to all of the RenderObjects first.)

These gestures use the new gesture detection system.