add service-worker-index.html

[nolanlawson]

This fixes #422 by outputting a new /service-worker-index.html file which can be cached by the service worker and served in offline use cases.

The reason I went with service-worker-index.html rather than the old index.html is that the latter breaks sapper export, since the service worker-targeted index.html conflicts with the base route. Since index.html is kind of an imprecise name for it anyway, I figured renaming to service-worker-index.html made the most sense.

The only change that users will have to make in order to get this to work is to use the commented code from sapper-template defined here. We'll probably want to update that commented code anyway, since it hasn't worked for about a year.

I have tested this with sapper-template and it works.

To test, you just npm link this PR to sapper-template, then in sapper-template, do npm run build && npm start and then navigate to the base URL, then put the Dev Tools in offline mode, uncomment the commented-out service worker code, then refresh. At this point, the new page should be served by the service worker using service-worker-index.html.

[nolanlawson]

This PR will fail in CI until #521 is merged.

[Conduitry]

Re-ran Travis build (the failing test looks like probably just some flakiness). I don't see how to and/or I don't have the permissions to re-run the Appveyor build.

[nolanlawson]

Rebased

[nolanlawson]

Side note: I am now running this in Pinafore unstable, at https://dev.pinafore.social. Been testing manually and so far everything seems to be working as before. 🙂

To be fair, though, Pinafore does not do anything fancy like preloading or server routes.

[nolanlawson]

Hmm, the prefetches programmatically test seems to be failing.

[nolanlawson]

I cannot reproduce the prefetches programmatically test failure locally. I'm using Ubuntu...

[Conduitry]

Take a look at #531 - if you haven't rebased on master since that was merged, give that a go.

[nolanlawson]

Rebased!

[nolanlawson]

Tests are green. :)

[Rich-Harris]

released 0.24.3, finally. thank you!