Shell "index.html" file is not created

[nolanlawson]

(Follow-up to #108)

Steps to repro:

git clone https://github.com/sveltejs/sapper-template.git
cd sapper-template
npm install
npm run build
npm start

Then go to localhost:3000/index.html in your browser.

Expected result: there is a shell HTML file, as described in this comment in the sapper-template service-worker.js.

Actual result: there is no shell HTML file, so the page 404s.

This makes it impossible AFAICT to build an actual offline-first Sapper app, i.e. one where the HTML page is cached by the Service Worker. Also this was working as of roughly January 2018, in the fork of Sapper I'm using for Pinafore.

I tested sapper export and it seems to have the same issue.