Repository: flutter
Issue: `DropdownButtonFormField` popupMenu invalid borderRadius
Labels: c: new feature,f: material design,c: proposal,P2,team-design,triaged-design

### Steps to reproduce

1. In `MaterialApp` specify 
```dart
      popupMenuTheme: PopupMenuThemeData(
        shape: RoundedRectangleBorder(
          borderRadius: BorderRadius.circular(12),
        ),
      ),
```

2.  create `DropdownButtonFormField`  with `borderRadius: BorderRadius.zero`

### Expected results

I expected popup get its borderRadius from theme (12).
It would be logical - all anchors (MenuAnchor, PopupMenuButton, etc) use borderRadius from popupMenuTheme.
But dropdown menu is different -_-

It seems like this menu is a lot of M2 menu - background, elevation, border...

### Actual results

Popup will be with radius 0 instead of 12

### Code sample

<details open><summary>Code sample</summary>

```dart
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        useMaterial3: true,
        popupMenuTheme: PopupMenuThemeData(
          shape: RoundedRectangleBorder(
            borderRadius: BorderRadius.circular(12),
          ),
        ),
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: Text(widget.title),
      ),
      body: Center(
        child: SizedBox(
          width: 100,
          child: DropdownButtonFormField(
            borderRadius: BorderRadius.zero,
            value: 1,
            items: const [
              DropdownMenuItem(value: 1, child: Text('1')),
              DropdownMenuItem(value: 2, child: Text('2')),
            ],
            onChanged: (_) {},
          ),
        ),
      ),
    );
  }
}
```

</details>


### Screenshots or Video

<details open>
<summary>Screenshots / Video demonstration</summary>

[Upload media here]
DropDown:


![Screenshot 2024-02-14 at 03 22 56](https://github.com/flutter/flutter/assets/78842812/816a0590-f4fa-4496-8bb7-330cf710778d)
![Screenshot 2024-02-14 at 03 23 02](https://github.com/flutter/flutter/assets/78842812/636fbadd-ff6a-45a3-8334-8e14ecae34b1)


Menu from `showMenu`:
![Screenshot 2024-02-14 at 03 26 15](https://github.com/flutter/flutter/assets/78842812/31956a39-9d44-46f7-aef0-6d6ddb0b0a3f)

</details>

### Flutter Doctor output

<details open><summary>Doctor output</summary>

```console
[✓] Flutter (Channel stable, 3.16.9, on macOS 14.3.1 23D60 darwin-arm64, locale en-RU)
    • Flutter version 3.16.9 on channel stable at /opt/homebrew/Caskroom/flutter/3.10.4/flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 41456452f2 (3 weeks ago), 2024-01-25 10:06:23 -0800
    • Engine revision f40e976bed
    • Dart version 3.2.6
    • DevTools version 2.28.5

[!] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
    • Android SDK at /Users/geara0/Library/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 15.2)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Build 15C500b
    • CocoaPods version 1.15.2

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2023.1)
    • Android Studio at /Users/geara0/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 2023.1.1)
    • IntelliJ at /Users/geara0/Applications/IntelliJ IDEA Ultimate.app
    • 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

[✓] VS Code (version 1.85.2)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.80.0

[✓] Connected device (2 available)
    • macOS (desktop) • macos  • darwin-arm64   • macOS 14.3.1 23D60 darwin-arm64
    • Chrome (web)    • chrome • web-javascript • Google Chrome 121.0.6167.160
    ! Error: Browsing on the local area network for iPhone HOME. Ensure the device is unlocked and attached with a cable or associated with the same local area network as this Mac.
      The device must be opted into Developer Mode to connect wirelessly. (code -27)
    ! Error: iPhone mrodeon has recently restarted. Xcode will continue when iPhone mrodeon is unlocked. (code -14)

[✓] Network resources
    • All expected network resources are available.

! Doctor found issues in 1 category.

```

</details>