Copied from unittest. Unittest will update to use this package once it has been published.

R=blois@google.com

Review URL: https://codereview.chromium.org//208823005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/matcher@34282 260f80e4-7a28-3924-810f-c04153c831b5