In dart-lang/language@ebd9e18, the ability to augment an enum value was removed. But the parser still accepts this code:

enum E {
  foo
}

augment enum E {
  augment foo
}

And the analyzer AST still has a slot for EnumConstantDeclaration.augmentKeyword.

Note that EnumConstantDeclaration.augmentKeyword is part of the analyzer public API, and is neither experimental nor deprecated, so to adhere strictly to semantic versioning, we should deprecate it and remove it in a future analyzer version bump.