Analyzer allows augmentations in standard library files

[DanTup]

The analyzer accepts this in a simple Dart library:

class A {}

augment class A {}

However the runtime rejects the augment keyword here.

I couldn't actually find it explicitly stated in https://github.com/dart-lang/language/blob/main/working/augmentation-libraries/feature-specification.md whether augmentations are allowed in normal libraries or only in library augmentations, but I think the intention is that they are only allowed in library augmentations, in which case the analyzer should be producing an error on augment.

[pq]

/fyi @scheglov