Definition
Service Workers Nightly defines ExtendableMessageEvent
[Exposed=ServiceWorker]
interface ExtendableMessageEvent : ExtendableEvent {
constructor(DOMString type, optional ExtendableMessageEventInit eventInitDict = {});
readonly attribute any data;
readonly attribute USVString origin;
readonly attribute DOMString lastEventId;
[SameObject] readonly attribute (Client or ServiceWorker or MessagePort)? source;
readonly attribute FrozenArray<MessagePort> ports;
};