[Web] deferred-loading "part" files not generated in Flutter 3.20.0+

[amake]

Steps to reproduce

1. With Flutter 3.19.4 generate a new project with flutter create my_project

2. Next to main.dart, introduce a new file foo.dart:

   int foo() => 1;

3. In main.dart, import foo as a deferred library and make trivial use of it:

   import 'package:flutter/material.dart';
   
   import 'foo.dart' deferred as foo; // new!
   
   void main() {
     runApp(const MyApp());
     foo.loadLibrary().then((_) => print(foo.foo())); // new!
   }
   
   // everything else unchanged from the default main.dart template

4. Run flutter build web and note that build/web/main.dart.js_1.part.js is generated

5. Upgrade to Flutter 3.20.0+

6. Run flutter clean && flutter build web and note that no *.part.js is generated. When deployed, the built application will fail due to missing the part files.

Expected results

The build/web/*.part.js files should be built as with Flutter 3.19 and earlier.

Actual results

The build/web/*.part.js files are not built.

Code sample

Code sample

main.dart

import 'package:flutter/material.dart';

import 'foo.dart' deferred as foo;

void main() {
  runApp(const MyApp());
  foo.loadLibrary().then((_) => print(foo.foo()));
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // TRY THIS: Try running your application with "flutter run". You'll see
        // the application has a purple toolbar. Then, without quitting the app,
        // try changing the seedColor in the colorScheme below to Colors.green
        // and then invoke "hot reload" (save your changes or press the "hot
        // reload" button in a Flutter-supported IDE, or press "r" if you used
        // the command line to start the app).
        //
        // Notice that the counter didn't reset back to zero; the application
        // state is not lost during the reload. To reset the state, use hot
        // restart instead.
        //
        // This works for code too, not just values: Most code changes can be
        // tested with just a hot reload.
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        useMaterial3: true,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // TRY THIS: Try changing the color here to a specific color (to
        // Colors.amber, perhaps?) and trigger a hot reload to see the AppBar
        // change color while the other colors stay the same.
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          //
          // TRY THIS: Invoke "debug painting" (choose the "Toggle Debug Paint"
          // action in the IDE, or press "p" in the console), to see the
          // wireframe for each widget.
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headlineMedium,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

foo.dart

int foo() => 1;

Screenshots or Video

Screenshots / Video demonstration

[Upload media here]

Logs

Logs

Executed command

{flutter -v clean && flutter -v build web && ls -al build/web} 2>&1 |pbcopy

Flutter 3.19

https://pastebin.com/aLtvFY2c

Flutter 3.20

https://pastebin.com/U7s4MJd7

Flutter Doctor output

Doctor output

Flutter 3.19

Doctor summary (to see all details, run flutter doctor -v):
[!] Flutter (Channel [user-branch], 3.19.4, on macOS 14.4 23E214 darwin-x64, locale en-JP)
    ! Flutter version 3.19.4 on channel [user-branch] at /Applications/flutter
      Currently on an unknown channel. Run `flutter channel` to switch to an official channel.
      If that doesn't fix the issue, reinstall Flutter by following instructions at https://flutter.dev/docs/get-started/install.
    ! Upstream repository unknown source is not a standard remote.
      Set environment variable "FLUTTER_GIT_URL" to unknown source to dismiss this error.
[!] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
    ✗ 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 15.3)
[✓] Chrome - develop for the web
[✓] Android Studio (version 2023.2)
[✓] IntelliJ IDEA Community Edition (version 2023.3.5)
[✓] Connected device (4 available)
[✓] Network resources

! Doctor found issues in 2 categories.

Flutter 3.20

Doctor summary (to see all details, run flutter doctor -v):
[!] Flutter (Channel [user-branch], 3.20.0-0.0.pre, on macOS 14.4 23E214 darwin-x64, locale en-JP)
    ! Flutter version 3.20.0-0.0.pre on channel [user-branch] at /Applications/flutter
      Currently on an unknown channel. Run `flutter channel` to switch to an official channel.
      If that doesn't fix the issue, reinstall Flutter by following instructions at https://flutter.dev/docs/get-started/install.
    ! Upstream repository unknown source is not a standard remote.
      Set environment variable "FLUTTER_GIT_URL" to unknown source to dismiss this error.
[!] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
    ✗ 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 15.3)
[✓] Chrome - develop for the web
[✓] Android Studio (version 2023.2)
[✓] IntelliJ IDEA Community Edition (version 2023.3.5)
[✓] Connected device (4 available)
[✓] Network resources

! Doctor found issues in 2 categories.

[danagbemava-nc]

Reproducible using the code sample and steps outlined above.

flutter doctor -v

[!] Flutter (Channel stable, 3.19.4, on macOS 14.3.1 23D60 darwin-arm64, locale en-GB)
    • Flutter version 3.19.4 on channel stable at /Users/nexus/dev/sdks/flutter
    ! Warning: `flutter` on your path resolves to /Users/nexus/dev/sdks/flutters/bin/flutter, which is not inside your current Flutter SDK checkout at /Users/nexus/dev/sdks/flutter. Consider adding /Users/nexus/dev/sdks/flutter/bin to the front of your path.
    ! Warning: `dart` on your path resolves to /Users/nexus/dev/sdks/flutters/bin/dart, which is not inside your current Flutter SDK checkout at /Users/nexus/dev/sdks/flutter. Consider adding /Users/nexus/dev/sdks/flutter/bin to the front of your path.
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 68bfaea224 (5 days ago), 2024-03-20 15:36:31 -0700
    • Engine revision a5c24f538d
    • Dart version 3.3.2
    • DevTools version 2.31.1
    • If those were intentional, you can disregard the above warnings; however it is recommended to use "git" directly to perform update checks and upgrades.

[✓] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
    • Android SDK at /Users/nexus/Library/Android/sdk
    • Platform android-34, build-tools 34.0.0
    • Java binary at: /Users/nexus/Applications/Android Studio.app/Contents/jbr/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 17.0.7+0-17.0.7b1000.6-10550314)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 15.3)
    • Xcode at /Applications/Xcode-15.3.0.app/Contents/Developer
    • Build 15E204a
    • CocoaPods version 1.14.3

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

[✓] Android Studio (version 2023.1)
    • Android Studio at /Users/nexus/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 17.0.7+0-17.0.7b1000.6-10550314)

[✓] IntelliJ IDEA Ultimate Edition (version 2023.2.5)
    • IntelliJ at /Users/nexus/Applications/IntelliJ IDEA Ultimate.app
    • Flutter plugin version 77.2.2
    • Dart plugin version 232.10286

[✓] VS Code (version 1.87.2)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.84.0

[✓] Connected device (3 available)
    • Dean’s iPad (mobile) • 00008103-000825C811E3401E • ios            • iOS 17.4.1 21E236
    • macOS (desktop)      • macos                     • darwin-arm64   • macOS 14.3.1 23D60 darwin-arm64
    • Chrome (web)         • chrome                    • web-javascript • Google Chrome 123.0.6312.58

[✓] Network resources
    • All expected network resources are available.

! Doctor found issues in 1 category.

[✓] Flutter (Channel master, 3.21.0-14.0.pre.9, on macOS 14.3.1 23D60 darwin-arm64, locale en-GB)
    • Flutter version 3.21.0-14.0.pre.9 on channel master at /Users/nexus/dev/sdks/flutters
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 7f06c263a4 (6 hours ago), 2024-03-25 00:33:34 -0400
    • Engine revision ae758d5463
    • Dart version 3.4.0 (build 3.4.0-268.0.dev)
    • DevTools version 2.34.0-dev.12

[✓] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
    • Android SDK at /Users/nexus/Library/Android/sdk
    • Platform android-34, build-tools 34.0.0
    • Java binary at: /Users/nexus/Applications/Android Studio.app/Contents/jbr/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 17.0.7+0-17.0.7b1000.6-10550314)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 15.3)
    • Xcode at /Applications/Xcode-15.3.0.app/Contents/Developer
    • Build 15E204a
    • CocoaPods version 1.14.3

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

[✓] Android Studio (version 2023.1)
    • Android Studio at /Users/nexus/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 17.0.7+0-17.0.7b1000.6-10550314)

[✓] IntelliJ IDEA Ultimate Edition (version 2023.2.5)
    • IntelliJ at /Users/nexus/Applications/IntelliJ IDEA Ultimate.app
    • Flutter plugin version 77.2.2
    • Dart plugin version 232.10286

[✓] VS Code (version 1.87.2)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.84.0

[✓] Connected device (4 available)
    • Dean’s iPad (mobile)            • 00008103-000825C811E3401E • ios            • iOS 17.4.1 21E236
    • macOS (desktop)                 • macos                     • darwin-arm64   • macOS 14.3.1 23D60 darwin-arm64
    • Mac Designed for iPad (desktop) • mac-designed-for-ipad     • darwin         • macOS 14.3.1 23D60 darwin-arm64
    • Chrome (web)                    • chrome                    • web-javascript • Google Chrome 123.0.6312.58

[✓] Network resources
    • All expected network resources are available.

• No issues found!

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