[cfe/ffi] pkg/front_end/test/ffi_test.dart

[dcharkes]

pkg/front_end/messages.yaml points to pkg/front_end/test/ffi_test.dart as its test, but we don't do any tests.

Instead of testing in the CFE way, we have tests expecting compile-time errors in tests/ffi/vmspecific_static_checks_test.dart.

We could consider adding CFE tests or making FFI messages exempt from that test flow.

[dcharkes]

2 Options proposed by @johnniwinther in https://dart-review.googlesource.com/c/sdk/+/249301

• I think we should just remove 'ffi_test.dart' and the external: test/ffi_test.dart entries in 'messages.yaml' and instead add an entry in 'messages.status' to allow the messages to have no example.
• Better yet, would be to add an example that triggers the message - this is the real benefit of the 'example:' entries in messages.yaml; that we can test that the exact message is triggered by the example. If it is not possible to trigger the message through an example, we should work towards making it possible.