main.dart

class Spacecraft {
  final String name;

  Spacecraft(this.name);
}

const oneSecond = Duration(seconds: 1);
// ···
Future<void> printWithDelay(String message) async {
  await Future.delayed(oneSecond);
  print(message);
}

Stream<String> report(Spacecraft craft, Iterable<String> objects) async* {
  for (final object in objects) {
    await Future.delayed(oneSecond);
    yield '${craft.name} flies by $object';
  }
}

void main() async {
  printWithDelay('hello.');

  const flybyObjects = ['Jupiter', 'Saturn', 'Uranus', 'Neptune'];

  final reportStream = report(Spacecraft('Voyager I'), flybyObjects);
  await for (final message in reportStream) {
    print(message);
  }
}