Incorporating nweiz' feedback.

BUG=
R=kustermann@google.com, sgjesse@google.com

Review URL: https://codereview.chromium.org//740323002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/unittest@41850 260f80e4-7a28-3924-810f-c04153c831b5