[cfe] co19/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t27 failure

[eernstg]

Said test causes a failure, cf. dart-lang/co19#1316. The failure is a missing compile-time error for a mixin that declares an operator == and has Enum as a superinterface (cf. https://github.com/dart-lang/language/blob/master/accepted/future-releases/enhanced-enums/feature-specification.md#implementing-enum-and-enum-types).

The test needs an adjustment as well, that will occur separately, dart-lang/co19#1319.

[eernstg]

Similar issue with co19/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t26 (in this case it's the other error message about nearly the same topic).

[eernstg]

Same issue with co19/LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t25.

[chloestefantsova]

I'm working on a fix at https://dart-review.googlesource.com/c/sdk/+/237580.

[eernstg]

A similar issue arises for hashCode in LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t23.

[eernstg]

Variant of the issue above with hashCode in LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t21.

[eernstg]

One more variant with hashCode in LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t19.

[eernstg]

One more variant with hashCode in LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t17.

[eernstg]

One more variant with hashCode in LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t15.

Fault in test handled separately, cf. dart-lang/co19#1321.

[eernstg]

One more variant in LanguageFeatures/Enhanced-Enum/implementing_enum_A04_t13.