Prerequisites (cc @jonahwilliams):

• migrate the framework [in progress by @a14n]
• publish platform/file/process in SDK allowlist, publish on pub, [in progress by @jonahwilliams]
• migrate flutter_goldens/flutter_goldens_client, Enable null safety for flutter_test #53908 [@jonahwilliams signed up for this]
• migrate flutter_test, Enable null safety for flutter_test #53908, [in progress by @goderbauer: Migrate flutter_test #66663]

It doesn't make sense to start migrating the tests before all the bullets above are done.

The long pole that will block flutter_test migration is the migration of the framework.