Mac no longer fire bogus events. Fix Mac watcher.

This is a re-apply of r37162.

BUG=
R=nweiz@google.com

Review URL: https://codereview.chromium.org//621813002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41220 260f80e4-7a28-3924-810f-c04153c831b5