should SW MessageEvents inherit ExtendableEvent?

[wanderview]

The spec has been changed so that most events fired on ServiceWorker inherit from ExtendableEvent and therefore have a .waitUntil() method. The one exception is MessageEvent. Should we have some kind of ExtendedMessageEvent that inherits from ExtendableEvent and MessageEvent? Right now one cannot do this:

addEventListener('message', function(event) {
  event.waitUntil(cache.add(event.data.request));
});

[jungkees]

Closing this issue in favor of #669. Please feel free to reopen it if you've intended something else.