[ServiceWorker] `waitUntil()` for FetchEvent? (#584)

AKA: should `FetchEvent` be an `ExtendableEvent`?

It's possible today to write code that fetches from the network, adds to a cache, and then matches from the cache in order to feed a document. This seems a bit weird...why invoke the Cache matching mechanism for a `Response` you've already got a handle to and can simply `.clone()`?

The answer, today, is the lifetime concern: you want the `respondWith` promise to extend beyond the point where the Cache is savvy to the cache entry. This is a bit weird. Mightn't it be better to give `FetchEvent` a `waitUntil()`?

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/584