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

@jakearchibald - This is really great! I like the flexibility of the proposal. I’m interested in understanding better if this will extend to more complex scenarios in the future.

- 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.
- Can this API handle racing sources rather than doing them in sequence?
- I think RouterIfResponse is an important feature, to allow for custom handling for error responses. However, things start to get more complicated with options that apply after the request has been made. For example, say you want to prefer network if the response is a 200, then try cache, then use the server response anyway if cached response is missing. Could you accomplish that with `add(new RouterSourceNetwork(), new RouterIfResponse());
add(new RouterSourceCache(), new RouterSourceNetwork())` without it issuing multiple network requests? Or, say you want to do the same thing but if either response isn’t a 200 then use custom logic to determine which to use. Would the fetch event be able to access the responses that were already requested via the routes?
- 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 can sort of see how you could do this with a route like `add(new RouterIfURLPrefix(), new RouterSourceNetwork())`, but you'd have to know which sub-resources will be requested. Could there be a RouterSource that causes the service worker to not control that client? Eg `add(new RouterIfURLPrefix(), new RouterSourceUnclaimClient())`. Or would that be too much of an abuse of the API?

Also one thought about implementation - 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.

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