code: function NotificationCenter(options) {
    this.logger = options.logger;
    this.errorHandler = options.errorHandler;
    this.__notificationListeners = {};
    fns.forOwn(enums.NOTIFICATION_TYPES, function(notificationTypeEnum) {
      this.__notificationListeners[notificationTypeEnum] = [];
    }.bind(this));
    this.__listenerId = 1;
  }
docs: NotificationCenter allows registration and triggering of callback functions using
  notification event types defined in NOTIFICATION_TYPES of utils/enums/index.js:
  - ACTIVATE: An impression event will be sent to Optimizely.
  - TRACK a conversion event will be sent to Optimizely
  @constructor
  @param {Object} options
  @param {Object} options.logger An instance of a logger to log messages with
  @param {object} options.errorHandler An instance of errorHandler to handle any unexpected error
  @returns {Object}