public void post(WatchEvent<?> event) {
      if (!events.offer(event)) {
        overflow.incrementAndGet();
      }
    }
Posts the given event to this key. After posting one or more events, {@link #signal()} must
be called to cause the key to be enqueued with the watch service.
jimfs/src/main/java/com/google/common/jimfs/AbstractWatchService.java