takeUntil<R>  method 
Emits values until an Observable emits a first value.
Implementation
Observable<T> takeUntil<R>(Observable<R> trigger) =>
    TakeUntilObservable<T, R>(this, trigger);Emits values until an Observable emits a first value.
Observable<T> takeUntil<R>(Observable<R> trigger) =>
    TakeUntilObservable<T, R>(this, trigger);