code: function eventData(emitter) {
  	return emitter[eventEmitter.EVENT_DATA_PROPERTY] || Object.defineProperty(emitter, eventEmitter.EVENT_DATA_PROPERTY, {
  		value: {},
  		enumerable: false
  	})[eventEmitter.EVENT_DATA_PROPERTY];
  }
docs: Get event data from an emitter. Lazily create it if it isn't there.