concatWith method
Returns an Observable that emits all items from the current Observable, then emits all items from the given observable, one after the next.
Example
new Observable.timer(1, new Duration(seconds: 10))
.concatWith([new Observable.just(2)])
.listen(print); // prints 1, 2
Implementation
Observable<T> concatWith(Iterable<Stream<T>> other) => new Observable<T>(
new ConcatStream<T>(<Stream<T>>[stream]..addAll(other)));