Add missing newline after Actual value in fail message.
Remove the BasMatcher class; it made some sense when we had both 'interface'
and 'class' but adds no value now.
Fix a comment in the CustomMatcher documentation.
Fix tests affected by these changes.

R=sigmund@google.com

Review URL: https://codereview.chromium.org//18442002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@25356 260f80e4-7a28-3924-810f-c04153c831b5