Address comments
Address comments

This addresses comments from several CLs:
* https://dart-review.googlesource.com/c/sdk/+/21461/4/pkg/front_end/lib/src/fasta/parser/parser.dart#1546
* https://dart-review.googlesource.com/c/sdk/+/21461/4/pkg/front_end/lib/src/fasta/parser/parser.dart#1590
* https://dart-review.googlesource.com/c/sdk/+/21463/3/pkg/front_end/lib/src/fasta/parser/parser.dart#1589
* https://dart-review.googlesource.com/c/sdk/+/21523/2/pkg/front_end/lib/src/fasta/parser/parser.dart#1096

Change-Id: I2fca69458b912ae5dcfb38ce86c1064f049d9bb2
Reviewed-on: https://dart-review.googlesource.com/22060
Reviewed-by: Peter von der Ahé <ad40f903ca639ca334d95dc5a5f0ae92bd708079@google.com>
Commit-Queue: Dan Rubel <5c5a7a1a3f9344592135bed55605519045cfa711@google.com>

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