[Enhanced enums] Compile-time errors on language/enum/enhanced_enums_basic_test

[alexmarkov]

Part of #48088.

Currently there are following compile-time errors on this test:

tests/language/enum/enhanced_enums_basic_test.dart:283:3: Error: Generative enum constructors must be marked as 'const'.
  EnumNoUnnamedConstructor.named(this.x);
  ^
tests/language/enum/enhanced_enums_basic_test.dart:293:3: Error: Generative enum constructors must be marked as 'const'.
  EnumFactoryUnnamedConstructor.named(this.x);
  ^
tests/language/enum/enhanced_enums_basic_test.dart:303:3: Error: Generative enum constructors must be marked as 'const'.
  Canonical(this.value);
  ^
tests/language/enum/enhanced_enums_basic_test.dart:317:21: Error: Enums can't contain declarations of members with the name 'values'.
  static String get values => "StaticDeclaration";
                    ^^^^^^
tests/language/enum/enhanced_enums_basic_test.dart:322:14: Error: Enums can't contain declarations of members with the name 'values'.
  String get values => "Declaration";
             ^^^^^^
tests/language/enum/enhanced_enums_basic_test.dart:325:6: Error: Can't declare a member that conflicts with an inherited one.
enum InheritsValues with ValuesMixin {
     ^^^^^^
tests/language/enum/enhanced_enums_basic_test.dart:369:14: Context: This is the inherited member.
  String get values => "Mixin";
             ^^^^^^
tests/language/enum/enhanced_enums_basic_test.dart:329:6: Error: Can't declare a member that conflicts with an inherited one.
enum ImplementsValues implements ValuesInterface {
     ^^^^^^
tests/language/enum/enhanced_enums_basic_test.dart:365:14: Context: This is the inherited member.
  String get values;
             ^^^^^^
tests/language/enum/enhanced_enums_basic_test.dart:309:11: Error: The return type of the method 'StringIndexEnum.index' is 'num', which does not match the return type, 'int', of the overridden method, '_Enum.index'.
Change to a subtype of 'int'.
  num get index => 3.5;

Repro:

tools/test.py -n dartk-strong-linux-release-x64 language/enum/enhanced_enums_basic_test

It looks like this test needs updating.

/cc @lrhn @eernstg

[eernstg]

Right, cf. https://dart-review.googlesource.com/c/sdk/+/233384.