code: function getListenerMap(target, opt_create) {
    let listenerMap = target.ol_lm;
    if (!listenerMap && opt_create) {
      listenerMap = target.ol_lm = {};
    }
    return listenerMap;
  }
docs: Get the lookup of listeners.
  @param {Object} target Target.
  @param {boolean=} opt_create If a map should be created if it doesn't exist.
  @return {!Object<string, Array<EventsKey>>} Map of
  listeners by event type.