Update exhaustiveness tests according to the language#3524

[sgrekhov]

Update exhaustiveness tests according to the dart-lang/language#3524
List of the tests that should be updated can be found here https://github.com/dart-lang/co19/pull/2437/files

[eernstg]

Thanks! The info needed is whether the language team wishes to have that always-exhaustive status propagate from an extension type erasure to the corresponding extension type. dart-lang/language#3524 should answer that question.

[sgrekhov]

Closing this issue. All of the tests mentioned in #2437 works on the latest SDK. Some of the tests were failing but they now are fixed by #2450