code: @SuppressWarnings("unused")
    private static synchronized void setTimerClass(String name,
        Class<? extends HeartbeatTimer> timerClass) {
      if (timerClass == null) {
        sTimerClasses.remove(name);
      } else {
        sTimerClasses.put(name, timerClass);
      }
    }
docs: Sets the timer class to use for the specified executor thread.
  
  This method should only be used by tests.
  
  @param name a name of a heartbeat executor thread
  @param timerClass the timer class to use for the executor thread