Re: [ServiceWorker] MessageEvent within the SW global should have waitUntil() (#669)

So we don't want it on the global because that encourages global reasoning rather than event-based reasoning, right? I think that's still applicable.

Perhaps we should just bite the bullet and introduce `ServiceWorkerMessageEvent`/`ClientMessageEvent`. With `ClientMessageEvent` (dispatched inside service workers) inheriting from `ExtendableEvent`.

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https://github.com/slightlyoff/ServiceWorker/issues/669#issuecomment-88860069