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
diff --git a/commits.json b/commits.json
index a6b9fdd..c80d8e3 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"c19459ab7d3f30c87a403aee6aadb483968c82ec",
   "external/github.com/flutter/flutter":"a1636b6fef2ebe7840ba6dc1b3f9d77095ef3b45",
-  "sdk":"77d2a59d8e76b2e2949138eed13de758f7869b29"
+  "sdk":"609c0d35eb84005a84c1de7d0c34c870dc509134"
 }
\ No newline at end of file