Re: [slightlyoff/ServiceWorker] Allow respondWith() to be called asynchronously or allow some way to cancel the response. (#836)

Sorry to necrobump guys:

@jakearchibald I agree that middleware is the right path for anyone thinking about multiple "respondWith"s, but that sidetracked this issue. A real problem right now  is that ```fetch(event.request)``` is not the same as simply not capturing the request, thanks to issues with range support and other outstanding issues with fetch.

@wanderview Afaik resolving a promise passed to respondWith to `undefined` or `null` is not part of the spec unless I'm mistaken, unless you mean passing null or undefined to respondWith directly. That seems like a simple solution to the problem, and actually the first thing I tried. I believe it currently has to be a response object or network error.

```javascript

self.addEventListener('fetch', event => {
  event.respondWith(async function () {
    const howToRespond = await howDoIRespond();
    if (howToRespond === 'fromCache') {
      return await getFromCache(event.request);
    } else if (howToRespond === 'fetch') {
      return await fetch(event.request);
    } else if (howToRespond === 'passthough') {
      // Oh no. I already called event.respondWith.
      // How do I cancel and let the browser do its normal thing?
    }
  });
});

```



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