I need help to generate test to fromEvent observable in angular6

in my code I have a function that uses fromEvent, that is an Observable

getNavigatorStatus(): Observable<any> {
  return merge(
    fromEvent(window, 'offline').pipe(map(() => false)),
    fromEvent(window, 'online').pipe(map(() => true)),
    Observable.create(sub => {
      sub.next(navigator.onLine);
      sub.complete();
    })
  );
}

and have this test:

it('getNavigatorStatus: should returns Observable', done => {
  spyOn(Observable, <any>'fromEvent').and.returnValue(of({}));
  spyOn(Observable, 'create').and.returnValue(of({}));

  NetworkService.getNavigatorStatus().subscribe(() => {
    expect(fromEvent).toHaveBeenCalledWith(window, 'offline');
    expect(fromEvent).toHaveBeenCalledWith(window, 'online');
    expect(Observable.create).toHaveBeenCalled();
    done();
  });
});

my problem is after migrating to angular 6, I can no longer spyOn fromEvent in the Observable like I used to do before angular 6.

spyOn(Observable, <any>'fromEvent').and.returnValue(of({}));

I need help to test the call of fromEvent (parameters, result, etc) inside my getNavigatorStatus function.