Breaking change: removed deprecated methods of PlatformMessages, leaving only binary messaging there. All other use of platform communication now goes through PlatformMessageChannel and PlatformMethodChannels. Retained use of String and JSON codecs for now.
Companion engine PR: flutter/engine#3482
* Bump to test `0.12.20`.
Some test `0.12.20` highlights:
* introduces `expectLater()` that returns a `Future` that completes when the matcher has finished running
* deprecates the `verbose` parameter to `expect()` and the `formatFailure()` (to be removed in `0.13.0`)
Otherwise:
* to keep up w/ the deprecation of `verbose`, removes `widget_tester` API to pass `verbose` flag (alternatively we could suppress the warning for now)
* Update stack manipulation.
* Fix framecount.
* Add find.descendant API to support find descendants of an element by passing the current element and the finder of descendant.
* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.
* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.
* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.
* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.
* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.
* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.
* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.
* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.
* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.
* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.
- output single-line JSON because iOS 9.x cannot output more than one line
- move the workaround for #7433 into LiveTestWidgetsFlutterBinding so that it works in all cases (e.g. animation_bench.dart on iOS)
Also, make hasOneLineDescription more discerning.
Also, add a test for hasOneLineDescription.
Also, add a test for GravitySimulation, to test the toString.
```dart
expect(x, moreOrLessEquals(0.0));
```
...for those cases where `x` might be 1e-11 or whatever.
Also, be more resilient when dumping the tree from inside the test framework.
Also, add an assert that helped me debug something the other day.
We were trying to update the tooltip overlay entry, but that cannot work
because the overlay entry might have already built. Instead, we keep the
old value.
Fixes#7151
* Remove the workaround that pinned args to v0.13.6
This reverts most of the changes in commit 6331b6c8b5
* throw exception if exit code is not an integer
* rework command infrastructure to throw ToolExit when non-zero exitCode
* convert commands to return Future<Null>
* cleanup remaining commands to use throwToolExit for non-zero exit code
* remove isUnusual exception message
* add type annotations for updated args package
