Changes in this patch:
- iOS now uses a different scrollDrag constant than Android.
- ScrollConfigurationDelegate now knows about target platforms.
- ScrollBehaviors now know about target platforms.
- RawInputLine now has to be told what platform it's targetting.
- PageableList now has a concept of target platform.
- make debugPrintStack filter its stack.
- move debugPrintStack to `assertions.dart`.
- add support for limiting the number of frames to debugPrintStack.
- make defaultTargetPlatform default to android in test environments.
- remove OverscrollStyle and MaterialApp's overscrollStyle argument. You
can now control the overscroll style using Theme.platform.
- the default scroll configuration is now private to avoid people
relying on the defaultTargetPlatform getter in their subclasses (since
they really should use Theme.of(context).platform).
- fix some typos I noticed in some tests.
- added a test for flinging scrollables, that checks that the behavior
differs on the two target platforms.
- made flingFrom and fling in the test API pump the frames.
- added more docs to the test API.
- made the TestAsyncUtils.guard() method report uncaught errors to help
debug errors when using that API.
