Upgrade unittest to be able to use isNotEmpty.

R=brianwilkerson@google.com, paulberry@google.com
BUG=

Review URL: https://codereview.chromium.org//790363006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42380 260f80e4-7a28-3924-810f-c04153c831b5