In service of https://github.com/flutter/flutter/issues/143348

This will make testing of asset transformation hot reload behavior easier (specifically in that we can avoid having to write a test that looks like this: https://github.com/flutter/flutter/blob/5d02c27248d9ebb0db79547f3ed0fce8060ff041/packages/flutter_tools/test/general.shard/build_system/targets/assets_test.dart#L167-L249