Returns a non-terminating observable sequence, which can be used to denote an infinite duration.
The never operator is one with very specific and limited behavior. These are useful for testing purposes, and sometimes also for combining with other Observables or as parameters to operators that expect other Observables as parameters.
new Observable.never().listen(print); // Neither prints nor terminates
factory Observable.never() => new Observable<T>(new NeverStream<T>());