Parse failure for broken augmentation library

[davidmorgan]

https://dart-review.googlesource.com/c/sdk/+/356401 adds a test case with a broken augmentation library, it trips an assert in the parse test

https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8754039752317012545/+/u/test_results/new_test_failures__logs_

'package:_fe_analyzer_shared/src/parser/parser_impl.dart': Failed assertion: line 846 pos 12: 'optional('augment', augmentKeyword)': is not true.

#0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:51:61)
#1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:40:5)
#2      Parser.parseLibraryAugmentation (package:_fe_analyzer_shared/src/parser/parser_impl.dart:846:12)
#3      Parser.parseTopLevelKeywordDeclaration (package:_fe_analyzer_shared/src/parser/parser_impl.dart:777:20)
#4      Parser.parseTopLevelDeclarationImpl (package:_fe_analyzer_shared/src/parser/parser_impl.dart:530:14)
#5      Parser.parseUnit (package:_fe_analyzer_shared/src/parser/parser_impl.dart:410:15)

I'll mark it approved since the failure is unimportant for the PR which adds test_runner support for expectations in augmentation libraries.