code: function(eventName, handler) {
      if (!this.__eventListeners) {
        this.__eventListeners = { };
      }
      // one object with key/value pairs was passed
      if (arguments.length === 1) {
        for (var prop in eventName) {
          this.on(prop, eventName[prop]);
        }
      }
      else {
        if (!this.__eventListeners[eventName]) {
          this.__eventListeners[eventName] = [ ];
        }
        this.__eventListeners[eventName].push(handler);
      }
    }
docs: Observes specified event
  @method observe
  @depracated Since 0.8.34. Use `on` instead.
  @param {String} eventName
  @param {Function} handler