Re: [w3c/ServiceWorker] Declarative routing (#1373)

@aaronsn 

> Would it be possible to have something like RouterIfTimeout which activates that route but cancels if the headers aren't received within a certain timeout? This would allow for preferring the network but falling back to cache if the network takes too long.

I think I'd make this an option to `RouterSourceNetwork`, as it's the only one that would benefit from a timeout right now.

> Can this API handle racing sources rather than doing them in sequence?

Something like:

```js
router.get(
  new RouterSourceAny([
    ...sources
  ])
);
```

This could also have a timeout option.

> I think RouterIfResponse is an important feature

I think everything you mention here is possible, although we might be hitting edge cases that are easier to read as logic in a fetch event, and uncommon enough that optimising them via a router doesn't bring much of a benefit.

> Avoiding controlling certain sub-scopes is something I'd like to see for service workers, but I'm not sure if this API can/should provide this support

I don't think it should. Only things you can currently do in a fetch event are in scope. To do this, we either want a way to exclude routes as part of the call to `serviceWorker.register`, or a way to do it via a fetch event (after which we could look at adding something to the router).

> if a request is handled via a static route, would the service worker still be started up in the background even though it isn't needed for the request? On the one hand, this would ensure that the service worker is started for future resource requests. On the other hand, if an entire page load (main resource plus sub resources) can be handled by static routes, then it's nice to avoid the performance cost of starting up the service worker.

Interesting! The spec is deliberately loose when it comes to when the service worker is started, and how long it stays alive for. Either behaviour would be spec compatible.

If a service worker is started, and not needed, it shouldn't affect page performance as nothing's blocked on it.

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