Add a missing import to pkg/watcher. R=rnystrom@google.com BUG= Review URL: https://codereview.chromium.org//104033003 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@30914 260f80e4-7a28-3924-810f-c04153c831b5