[cfe] Error when omitting base/final/sealed on base/final superclasses through legacy libraries.

[kallentu]

https://github.com/dart-lang/sdk/blob/main/tests/language/class_modifiers/trans_legacy/legacy_superdeclaration_error_test.dart
Missing errors from line 46 and onwards.

The CFE is not reporting an error when a class from a 3.0 library is a subclass of a class in a 2.19 library whose supertype is from the core libraries.

[kallentu]

Done through https://dart-review.googlesource.com/c/sdk/+/301503.

[kallentu]

@sgrekhov
versioning_A03_t01.dart - checks that final class cannot be extended
versioning_A03_t04.dart - checks that mixin on a final class cannot be declared

These two should be fixed with this fix.

[sgrekhov]

Yes, now it works. Confirmed. Thank you