code: function (newListeners, existingListeners) {
          if (newListeners != null) {
              if (!existingListeners) {
                  existingListeners = [];
              }
              existingListeners.push.apply(existingListeners, newListeners);
          }
          return existingListeners;
      }
docs: Append the given new array of listeners to the existing array of listeners. It is used when retrieving the array
  of listeners to call. The resulting array of listeners can be modified without changing newListeners. If both
  element are null, will return null
  @param {Array} newListeners listeners to add, can be null
  @param {Array} existingListeners existing listeners, can be null
  @return {Array}
  @private