Use filtered stacks in pub, barback, and watcher tests.

This also exposes more unittest stuff to users of scheduled_test.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//23582003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@26751 260f80e4-7a28-3924-810f-c04153c831b5