Refactoring in some *_contributor_test.dart files, more assert*() method declarations to superclass to share instead of having multiple copies

Change-Id: I43b539c78b5b3880b747c75add1f82ca78510893
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/149365
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Auto-Submit: Jaime Wren <jwren@google.com>

https://dart.googlesource.com/sdk/+/609c0d35eb84005a84c1de7d0c34c870dc509134
1 file changed
tree: 671db04ef32c833dad6a72ead7448ccaf2d89140
  1. commits.json