Most of these imports were never appropriate. The `test_api` package was never intended for use in `_test.dart` files.
Where possible move imports to `matcher`, otherwise move them to `test` or `flutter_test`.

Leave uses of `test_api` from `flutter_test` library code.

This reverts commit d056500b.

This reverts commit 31de052e.

This reverts commit e2490f29.

This reverts commit 7ba775a9.

Move Flutter Gallery to dev/integration_tests/ as it is an older copy used only for testing. The current version of the Flutter Gallery now lives in https://github.com/flutter/gallery.