Update pubspec yaml, so the transformer only runs on the intended tests.
Update pubspec yaml, so the transformer only runs on the intended tests.

BUG=
R=eernst@google.com

Review URL: https://codereview.chromium.org/1132043003

jakemac53/reflectable,luisvt/reflectable,jakemac53/reflectable,luisvt/reflectable,google/reflectable.dart,google/reflectable.dart,bwu-dart-contributing/reflectable,bwu-dart-contributing/reflectable