Re: [ServiceWorker] Can we replace event.default() with something less magic? (#607)

@annevk It looks like if I wait to check the cache before calling `event.respondWith()` then it's too late:

`Uncaught (in promise) DOMException: Failed to execute 'respondWith' on 'FetchEvent': The fetch event has already been responded to.`

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https://github.com/slightlyoff/ServiceWorker/issues/607#issuecomment-118370014