This CL updates Dart.g and hence the spec parser to handle new features
added to Dart in 2.17.
Change-Id: I42d6534d933df9a30453f12643dcc90e4d86fa80
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/260821
Reviewed-by: William Hesse <whesse@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Erik Ernst <eernst@google.com>