*     ordered by startup time.
   */
  public ImmutableMap<Service, Long> startupTimes() {
    return state.startupTimes();
  }

  @Override
  public String toString() {
    return MoreObjects.toStringHelper(ServiceManager.class)
        .add("services", Collections2.filter(services, not(instanceOf(NoOpService.class))))
        .toString();
  }

  /**