This changes the way the 'augment' modifier is handled in the parser.
Instead of being special cased for class declarations, it is now a
built-in identifier recognized only when the scanner is configured to
support augmentation libraries.
The change supports the used of the 'augment' modifier on not only
class declarations but also mixin and member declarations, as needed
for supporting macros.
Change-Id: I2679d198dea360900cfc478fae9c1660702a8a02
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/233887
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Jens Johansen <jensj@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Johnni Winther