[Augmentations] No error in the analyzer in case of augmented declaration

[sgrekhov]

According to the specification

A compile-time error occurs if a declaration with the basename augmented occurs in a location where the outermost enclosing declaration is augmenting.

But analyzer doesn't report this error. See for example https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Augmentation-libraries/augmented_expression_A09_t01_lib.dart (and any other test named augmented_expression_A09_*.dart)