src/main/java/io/reactivex/internal/operators/flowable/FlowableLastSingle.java
@Override
    protected void subscribeActual(SingleObserver<? super T> observer) {
        source.subscribe(new LastSubscriber<T>(observer, defaultItem));
    }
@Override
    protected void subscribeActual(SingleObserver<? super T> observer) {
        source.subscribe(new LastSubscriber<T>(observer, defaultItem));
    }
TODO fuse back to Flowable