This reverts commit 36a0bc5b.

Makes the android_semantics_testing test not fail for newer versions of Talkback.

Also, since the test now depends on pub_semver, upgraded the pubspec files.

The only substantive changes in the PR are in dev/integration_tests/android_semantics_testing/test_driver/main_test.dart

This reverts commit 5fd259be.

This reverts commit cb378edc.

* Add a microbenchmark that measures performance of flutter compute.

This benchmark should show performance improvements when compute is switched to Isolate.exit

* Fix analyzer warnings, increase warmup iterations

* Fix license header

This adds a smoke test for every single API example. It also fixes 17 tests that had bugs in them, or were otherwise broken, and even fixes one actual bug in the framework, and one limitation in the framework.

The bug in the framework is that NetworkImage's _loadAsync method had await response.drain<List<int>>();, but if the response is null, it will throw a cryptic exception saying that Null can't be assigned to List<int>. The fix was just to use await response.drain<void>(); instead.

The limitation is that RelativePositionedTransition takes an Animation<Rect> rect parameter, and if you want to use a RectTween with it, the value emitted there is Rect?, and one of the examples was just casting from Animation<Rect> to Animation<Rect?>, which is invalid, so I modified RelativePositionedTransition to take a Rect? and just use Rect.zero if the rect is null.