Since abb36e3cb, we no longer rely on the cli to set up ServiceWorker, but do it
manually as part of `yarn build`. When using `ng serve`, registering the
ServiceWorker fails, because we haven't created `ngsw-manifest.json` nor copied
`worker-basic.min.js` into dist.
This commit works around this, by only registering the service worker in
production mode (which is what the cli does too).
Caveat:
It is not possible to enable ServiceWorker with `ng serve`/`yarn start` and
using the `--prod` flag will try to register it, but fail because the necessary
files (`ngsw-manifest.json` and `worker-basic.min.js`) will not be available.
(As a work-around, you can use `yarn build` and serve the files in `dist/` with
`yarn http-server -- dist -p 4200`.)
90bc5b221b