function CallbackQueue(arg) {
    _classCallCheck(this, CallbackQueue);

    this._callbacks = null;
    this._contexts = null;
    this._arg = arg;
  }
DOMProperty exports lookup objects that can be used like functions:

  > DOMProperty.isValid['id']
  true
  > DOMProperty.isValid['foobar']
  undefined

Although this may be confusing, it performs better in general.

@see http://jsperf.com/key-exists
@see http://jsperf.com/key-missing