language/covariant/subtyping_test is failing

[liamappelbe]

language/covariant/subtyping_test is failing in NNBD weak or strong mode. I diffed this test with the language_2 version (which passes in weak mode), and I don't see anything that would legitimately cause the compile errors I'm seeing, so I think it's a CFE issue.

Logs:
https://dart-ci.appspot.com/log/vm-kernel-nnbd-linux-debug-x64/dartk-weak-asserts-linux-debug-x64/1064/language/covariant/subtyping_test