Re: [slightlyoff/ServiceWorker] Eliminating SW startup latency for common case (#920)

Aside from Facebook problem. Although @jakearchibald declarative syntax is tempting, I think it is too soon for this. @slightlyoff preflight solution seems to me a very valid options but I really think it should be opt-in so, during registration, a service worker could include a preflight list (`networkRace`) such as:

```js
navigator.serviceWorker.register('sw.js', { scope: '/', networkRace: ['/ping', '/shell-assets'] });
```

List items would act as prefixes so any request to `/shell-assets/*` would be raced with the service worker. About the problem with the 3 GiB video, you could cancel the response in progress (I'm not talking about _cancellable_ promises) by modifying the API:

```js
self.onfetch = (e) => {
  if (isHeavyAsset(e.request) && e.networkResponse.inProgress) {
    e.networkResponse.abort();
    e.respondWith(handle(e.request));
  }
}
```

If for some race condition, you end sending another request to the network as in:

```js
self.onfetch = event => {
  event.respondWith(
    event.preflightResponse || fetch(event.request)
  )
}
```

The second one will hit the http cache hopefully instead of reaching the actual network.

Does it make sense?

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