combine2<A, B, R>  static method 
Constructs a CombineLatestStream from a pair of Streams
where combiner is used to create a new event of type R, based on the
latest events emitted by the provided Streams.
Implementation
static CombineLatestStream<dynamic, R> combine2<A, B, R>(
  Stream<A> streamOne,
  Stream<B> streamTwo,
  R Function(A a, B b) combiner,
) =>
    CombineLatestStream<dynamic, R>(
      [streamOne, streamTwo],
      (List<dynamic> values) => combiner(values[0] as A, values[1] as B),
    );