When not dismissing, the Dismissable widget should cull its background.
When a dismiss is in progress, it should clip the background to just the
part that is revealed.

Fixes #6127

The code previously assumed that paint would be called before hitTest, but that
assumption isn't valid because RenderOpacity hit tests its children without
painting them.

Fixes #6089

Turns out that previously we weren't updating the clip if the layout
didn't change, even if the delegate was different.

This makes it possible to substitute 'flutter run' for 'flutter test'
and actually watch a test run on a device.

For any test that depends on flutter_test:

1. Remove any import of 'package:test/test.dart'.

2. Replace `testWidgets('...', (WidgetTester tester) {`
      with `testWidgets('...', (WidgetTester tester) async {`

3. Add an "await" in front of calls to any of the following:
    * tap()
    * tapAt()
    * fling()
    * flingFrom()
    * scroll()
    * scrollAt()
    * pump()
    * pumpWidget()

4. Replace any calls to `tester.flushMicrotasks()` with calls to
   `await tester.idle()`.

There's a guarding API that you can use, if you have particularly
complicated tests, to get better error messages. Search for
TestAsyncUtils.

The `flutter` package gets more types!

Follow-up from: https://github.com/flutter/flutter/pull/3727.

* ClipPath

* Add a test for ClipOval and ClipPath