Create ListenableFuture<Pair<A, B>> from ListenableFuture<A> and ListenableFuture<B>

transformAsyncPair.kt

fun <A, B> transformAsyncPair(futureA: ListenableFuture<A>, futureB: ListenableFuture<B>, executor: Executor): ListenableFuture<Pair<A, B>> {
    return Futures.transformAsync(futureA, AsyncFunction<A, Pair<A, B>> { a ->
        Futures.transformAsync(futureB, AsyncFunction<B, Pair<A, B>> { b ->
            Futures.immediateFuture(Pair<A, B>(a!!, b!!))
        }, executor)
    }, executor)
}