Re-implement directory polling. BUG=https://code.google.com/p/dart/issues/detail?id=12107 R=nweiz@google.com Review URL: https://codereview.chromium.org//21628002 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@25746 260f80e4-7a28-3924-810f-c04153c831b5