windowTime method
Creates a Stream where each item is a Stream containing the items from
the source sequence, sampled on a time frame with duration.
Example
Stream.periodic(Duration(milliseconds: 100), (int i) => i)
  .windowTime(Duration(milliseconds: 220))
  .doOnData((_) => print('next window'))
  .flatMap((s) => s)
  .listen(print); // prints next window 0, 1, next window 2, 3, ...
Implementation
Stream<Stream<T>> windowTime(Duration duration) =>
    window(Stream<void>.periodic(duration));