Repository: flutter
Issue: Improve "DropdownButton's selectedItemBuilder" assertion exception 
Labels: framework,f: material design,a: error message,team-design,triaged-design

Run this:

```dart
import 'package:flutter/material.dart';

void main() {
  runApp(
    MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData.dark(),
      home: Scaffold(
        body: DropdownButtonHideUnderline(
          child: DropdownButtonFormField<Color>(
            selectedItemBuilder: (context) {
              return [
                DropdownMenuItem<Color>(
                  value: Colors.green,
                  child: RichText(
                    text: TextSpan(
                      children: [
                        WidgetSpan(
                          child: Padding(
                            padding: const EdgeInsets.only(
                              right: 10,
                            ),
                            child: Icon(Icons.ac_unit),
                          ),
                          baseline: TextBaseline.alphabetic,
                          alignment: PlaceholderAlignment.aboveBaseline,
                        ),
                        const TextSpan(text: 'By '),
                        TextSpan(
                          text: 'test',
                          style: const TextStyle(
                            fontWeight: FontWeight.w700,
                          ),
                        ),
                      ],
                    ),
                  ),
                )
              ];
            },
            icon: SizedBox(),
            value: Colors.accents[1],
            decoration: const InputDecoration(
              enabledBorder: UnderlineInputBorder(
                borderSide: BorderSide(
                  color: Colors.transparent,
                ),
              ),
              focusedBorder: UnderlineInputBorder(
                borderSide: BorderSide(
                  color: Colors.transparent,
                ),
              ),
              labelStyle: TextStyle(
                color: Colors.black,
                fontWeight: FontWeight.w500,
                fontSize: 14,
              ),
            ),
            items: Colors.accents.map((value) {
              return DropdownMenuItem<Color>(
                value: value,
                child: RichText(
                  text: TextSpan(
                    children: [
                      TextSpan(
                        text: value.toString(),
                        style: const TextStyle(
                          fontWeight: FontWeight.w700,
                        ),
                      ),
                    ],
                  ),
                ),
              );
            }).toList(),
            onChanged: (value) {},
          ),
        ),
      ),
    ),
  );
}
```

Doctor:
```
[✓] Flutter (Channel stable, 2.5.3, on Microsoft Windows [Version 10.0.22000.282], locale en-US)
    • Flutter version 2.5.3 at C:\flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 18116933e7 (2 weeks ago), 2021-10-15 10:46:35 -0700
    • Engine revision d3ea636dc5
    • Dart version 2.14.4

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
    • Android SDK at C:\Users\Agone\AppData\Local\Android\sdk
    • Platform android-30, build-tools 30.0.3
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7249189)
    • All Android licenses accepted.

[✓] Visual Studio - develop for Windows (Visual Studio Community 2019 16.11.5)
    • Visual Studio at C:\Program Files (x86)\Microsoft Visual Studio\2019\Community
    • Visual Studio Community 2019 version 16.11.31729.503
    • Windows 10 SDK version 10.0.19041.0

[✓] Android Studio (version 2020.3)
    • Android Studio at C:\Program Files\Android\Android Studio
    • 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-b96-7249189)

[✓] IntelliJ IDEA Ultimate Edition (version 2021.2)
    • IntelliJ at C:\Program Files\JetBrains\IntelliJ IDEA 2021.2.2
    • 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 1.61.2)
    • VS Code at C:\Users\Agone\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.27.0

[✓] VS Code (version 1.62.0-insider)
    • VS Code at C:\Users\Agone\AppData\Local\Programs\Microsoft VS Code Insiders
    • Flutter extension version 3.27.0

[✓] Connected device (1 available)
    • Windows (desktop) • windows • windows-x64 • Microsoft Windows [Version 10.0.22000.282]

• No issues found!
```

Selected item disappears as a result.