Take a broader approach to filtering out bogus Mac OS watcher events.

With luck, this should unflake the test.

R=rnystrom@google.com
BUG=16079

Review URL: https://codereview.chromium.org//140013002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@31853 260f80e4-7a28-3924-810f-c04153c831b5