Observables didn't properly integrate with promises at the right time - if they did they'd be accepted way more easily (e.g. `.first()` returning a Promise rath

Observables didn't properly integrate with promises at the right time - if they did they'd be accepted way more easily (e.g. `.first()` returning a Promise rather than an observable or `using` accepting promise return values and/or async functions and returning promises as a result). I gave the Dart API originally as an example, as it has careful considerations for when to return Future<T> instead of another Stream<T> (https://api.dart.dev/stable/2.18.7/dart-async/Stream-class.html https://api.dart.dev/stable/2.18.7/dart-async/Stream-class.h...) - examples include first(), last(), asyncMap etc.