[Augmentations] Analyzer reports an error when augmentation provides no default values

[sgrekhov]

According to the specification

It is a compile-time error if, after type inheritance:
...

• The augmenting function specifies any default values. Default values are defined solely by the original function.

But the analyzer reports an error if the augmenting function doesn't specify default values. See, for example, https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Augmentation-libraries/augmented_expression_A04_t26_lib.dart