Nested async generator doesn't work as expected

[omatt]

Steps to Reproduce

Minimal repro

import 'dart:async';

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Unexpected Behavior with Nested Async Generators in Dart',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: CounterWidget(),
    );
  }
}

class CounterWidget extends StatefulWidget {
  CounterWidget({Key? key}) : super(key: key);

  @override
  _CounterWidgetState createState() => _CounterWidgetState();
}

class _CounterWidgetState extends State<CounterWidget> {
  var correct = <int>[];
  var wrong = <int>[];

  final blocCorrect = Counter();
  final blocWrong = Counter();

  @override
  void initState() {
    super.initState();
    blocCorrect.listen((i) {
      correct.add(i);
      setState(() {});
    });

    blocWrong.listen((i) {
      wrong.add(i);
      setState(() {});
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text(
              correct.toString(),
              style: TextStyle(color: Colors.black),
            ),
            SizedBox(height: 50),
            TextButton(
              child: Text('Correct'),
              onPressed: () => blocCorrect.add(CounterEvent.correct),
            ),
            SizedBox(height: 50),
            Container(
              color: Colors.black,
              width: 200,
              height: 2,
            ),
            SizedBox(height: 50),
            Text(
              wrong.toString(),
              style: TextStyle(color: Colors.black),
            ),
            SizedBox(height: 50),
            TextButton(
              child: Text('Wrong'),
              onPressed: () => blocWrong.add(CounterEvent.wrong),
            ),
          ],
        ),
      ),
    );
  }
}

enum CounterEvent { correct, wrong }

class Counter extends Bloc<CounterEvent, int> {
  Counter() : super(0);

  @override
  Stream<int> mapEventToState(CounterEvent event) async* {
    if (event == CounterEvent.correct) {
      // Works
      yield state + 1;
      yield state + 1;
    } else {
      /// Does not work
      yield* _increment();
    }
  }

  Stream<int> _increment() async* {
    yield state + 1;
    yield state + 1;
  }
}

abstract class Bloc<E, S> {
  Bloc(this._state) {
    _eventController.stream.asyncExpand(mapEventToState).listen((s) {
      _state = s;
      _stateController.add(_state);
    });
  }

  final _eventController = StreamController<E>();
  final _stateController = StreamController<S>.broadcast();

  S _state;
  S get state => _state;

  StreamSubscription<S> listen(void Function(S) onData) {
    return _stateController.stream.listen(onData);
  }

  Stream<S> mapEventToState(E event);

  void add(E event) => _eventController.add(event);
}

1. Run minimal repro
2. Click 'Correct', prints 1,2
3. Click 'Wrong', prints 1,1

Expected results:

Clicking 'Wrong' should print 1,2 - similar to the output of clicking 'Correct'

Actual results:

yield state doesn't get update as expected. Clicking 'Wrong' prints 1,1

As a workaround, _increment() can be made recursive. Calling yield* _increment(2) prints 1,2

Stream<int> _increment(int n) async* {
  if (n > 0) {
    yield state + 1;
    yield* _increment(n - 1);
  }
}

Logs

[✓] Flutter (Channel stable, 2.5.1, on macOS 11.6 20G165 darwin-x64, locale en)
    • Flutter version 2.5.1
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision ffb2ecea52 (2 weeks ago), 2021-09-17 15:26:33 -0400
    • Engine revision b3af521a05
    • Dart version 2.14.2

[!] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
    • Android SDK
    ✗ cmdline-tools component is missing
      Run `path/to/sdkmanager --install "cmdline-tools;latest"`
      See https://developer.android.com/studio/command-line for more details.
    ✗ Android license status unknown.
      Run `flutter doctor --android-licenses` to accept the SDK licenses.
      See https://flutter.dev/docs/get-started/install/macos#android-setup for more details.

[✓] Xcode - develop for iOS and macOS
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.0.1, Build version 12A7300
    • CocoaPods version 1.10.0

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2020.3)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7281165)

[✓] VS Code (version 1.60.2)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.26.0

[✓] Connected device (1 available)
    • Chrome (web) • chrome • web-javascript • Google Chrome 94.0.4606.61

! Doctor found issues in 1 category.

[omatt]

Moving this to dart-lang/sdk#47355

[github-actions]

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