fix(aio): only register ServiceWorker in production mode

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`.)

aio/README.md

@@ -31,6 +31,18 @@ Here are the most important tasks you might need to use:
 * `yarn generate-zips` - generate the zip files from the examples. Zip available via the `live-example` tags in the docs.
 
 
+## Using ServiceWorker locally
+
+Since abb36e3cb, running `yarn start -- --prod` will no longer set up the ServiceWorker, which
+would require manually running `yarn sw-manifest` and `yarn sw-copy` (something that is not possible
+with webpack serving the files from memory).
+
+If you want to test ServiceWorker locally, you can use `yarn build` and serve the files in `dist/`
+with `yarn http-server -- dist -p 4200`.
+
+For more details see #16745.
+
+
 ## Guide to authoring
 
 There are two types of content in the documentatation:

aio/src/main.ts

@@ -9,7 +9,7 @@ if (environment.production) {
 }
 
 platformBrowserDynamic().bootstrapModule(AppModule).then(ref => {
-  if ('serviceWorker' in (navigator as any)) {
+  if (environment.production && 'serviceWorker' in (navigator as any)) {
     const appRef: ApplicationRef = ref.injector.get(ApplicationRef);
     appRef.isStable.first().subscribe(() => {
       (navigator as any).serviceWorker.register('/worker-basic.min.js');