* Add a `matchesGoldenFile()` async matcher that will match
a finder's widget's rasterized image against a golden file.
* Add support for pluggable image comparison backends
* Add a default backend that does simplistic PNG byte
comparison on locally stored golden files.
* Add support for `flutter test --update-goldens`, which will
treat the rasterized image bytes produced during the test
as the new golden bytes and update the golden file accordingly
Still TODO:
* Add support for the `flutter_test_config.dart` test config hook
* Utilize `flutter_test_config.dart` in `packages/flutter/test`
to install a backend that retrieves golden files from a dedicated
`flutter/goldens` repo
https://github.com/flutter/flutter/issues/16859
