See flutter/packages#11001 (comment) for context. Each copy of the iOS implementation package has a custom test that checks that its source matches the shared source, but if a PR doesn't touch any files in one of the copies, then that copy doesn't run any tests (because of the way the repo tool targets changed packages), and so nothing fails.

I think the easiest approach is probably to just have each test verify all the copies. That's duplicate work, but a small enough amount of work that it's probably not worth optimizing.