Mac no longer fire bogus events. Fix Mac watcher. BUG= R=nweiz@google.com Review URL: https://codereview.chromium.org//319433003 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@37162 260f80e4-7a28-3924-810f-c04153c831b5