Wait until the end of the microtask to tell gesture recognizers that
they've won in the gesture arena. This lets recognizers dispose reject
themselves at arbitrary times without triggering gestures in awkward
call stacks.
Fixes#3183
Moves TestGesture into test_pointer.dart and makes it more
self-contained.
This is part of a general refactoring of flutter_test.
Depends on https://github.com/flutter/flutter/pull/3459
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.
