ServiceWorker never installs if a timer is running

[CarstenLeue]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[x] Bug report  
[ ] Performance issue
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question
[ ] Other... Please describe:

Current behavior

When registering a service worker via ServiceWorkerModule.register('/ngsw-worker.js') it turns out that this worker will only be registered once the application is considered "stable".
This in turn is determined by checking if there are outstanding macro- or microtasks.

If the application e.g. uses a interval timer (e.g. to periodically refresh itself), then the service worker will never be registered, because the application never reaches its stable state.

Expected behavior

I would expect that it is supported to use timers and intervals in the application and that these do NOT cause a service worker from not being registered.

Minimal reproduction of the problem with instructions

Use an empty angular app, add service worker support via ng add @angular/pwd and start a timer in the constructor of your module. As a result the service worker will never be registered.

Environment

Angular version: 6.1.2

[gkalpak]

Duplicate of #20970.

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