[augmentations] No error if there is metadata named augmented in augmentation scope

[sgrekhov]

In an augmentation scope augmented is a reserved world and can be used for calling introductory declaration only. Any other use of this word is a compile-time error. But there is no expected error in the analyzer below:

class augmented {
  const augmented();
}
class C {}

augment class C {
  @augmented()  // There should be an error here but there isn't
  void foo() {}
}

Appropriate co19 test, for example, https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Augmentation-libraries/augmented_expression_A03_t33.dart