Update the Mac OS watcher in pkg/watcher. The behavior of Directory.watch has changed, and this updates the watcher to match the new behavior. R=rnystrom@google.com BUG= Review URL: https://codereview.chromium.org//100823005 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@30915 260f80e4-7a28-3924-810f-c04153c831b5