ExtendableEvent(String type, [Map eventInitDict])

Source 

@DomName('ExtendableEvent.ExtendableEvent')
@DocsEditable()
factory ExtendableEvent(String type, [Map eventInitDict]) {
  if (eventInitDict != null) {
    var eventInitDict_1 = convertDartToNative_Dictionary(eventInitDict);
    return _blink.BlinkExtendableEvent.instance.constructorCallback_2_(type, eventInitDict_1);
  }
  return _blink.BlinkExtendableEvent.instance.constructorCallback_1_(type);
}