Repository: flutter
Issue: ReordableList itemExtent and prototypeItem are broken
Labels: framework,f: material design,f: scrolling,a: quality,has reproducible steps,P2,found in release: 2.3,team-design,triaged-design

## Steps to Reproduce

1. Create ReordableList with either itemExtent or prototypeItem specified
2. Drag the item

**Expected results:** No additional offset on drag

**Actual results:** Additional offset on drag

<details>
  <summary>Code sample</summary>

```dart
/// Flutter code sample for ReorderableListView

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

/// This is the main application widget.
class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: Scaffold(
        appBar: AppBar(title: const Text(_title)),
        body: const MyStatefulWidget(),
      ),
    );
  }
}

/// This is the stateful widget that the main application instantiates.
class MyStatefulWidget extends StatefulWidget {
  const MyStatefulWidget({Key? key}) : super(key: key);

  @override
  State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}

/// This is the private State class that goes with MyStatefulWidget.
class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  final List<int> _items = List<int>.generate(50, (int index) => index);

  @override
  Widget build(BuildContext context) {
    final ColorScheme colorScheme = Theme.of(context).colorScheme;
    final Color oddItemColor = colorScheme.primary.withOpacity(0.05);
    final Color evenItemColor = colorScheme.primary.withOpacity(0.15);

    return ReorderableListView(
      itemExtent: 48.0,
      padding: const EdgeInsets.symmetric(horizontal: 40),
      children: <Widget>[
        for (int index = 0; index < _items.length; index++)
          ListTile(
            key: Key('$index'),
            tileColor: _items[index].isOdd ? oddItemColor : evenItemColor,
            title: Text('Item ${_items[index]}'),
          ),
      ],
      onReorder: (int oldIndex, int newIndex) {
        setState(() {
          if (oldIndex < newIndex) {
            newIndex -= 1;
          }
          final int item = _items.removeAt(oldIndex);
          _items.insert(newIndex, item);
        });
      },
    );
  }
}

```

</details>

<details>
  <summary>flutter doctor -v</summary>

```
[√] Flutter (Channel master, 2.3.0-17.0.pre.397, on Microsoft Windows [Version 10.0.19041.1052], locale ru-RU)
    • Flutter version 2.3.0-17.0.pre.397 at C:\dev\src\flutter
    • Upstream repository git@github.com:nt4f04und/flutter.git
    • Framework revision 03034e9e4a (25 hours ago), 2021-06-18 00:14:03 -0700
    • Engine revision ced58ef9c1
    • Dart version 2.14.0 (build 2.14.0-217.0.dev)

[!] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
    X cmdline-tools component is missing
      Run `path/to/sdkmanager --install "cmdline-tools;latest"`
      See https://developer.android.com/studio/command-line for more details.
    X Android license status unknown.
      Run `flutter doctor --android-licenses` to accept the SDK licenses.
      See https://flutter.dev/docs/get-started/install/windows#android-setup for more details.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe

[√] Visual Studio - develop for Windows (Visual Studio Community 2019 16.7.7)
    • Visual Studio at C:\Program Files (x86)\Microsoft Visual Studio\2019\Community
    • Visual Studio Community 2019 version 16.7.30621.155
    • Windows 10 SDK version 10.0.19041.0

[√] Android Studio (version 4.1.0)
    • 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 1.8.0_242-release-1644-b01)

[√] IntelliJ IDEA Community Edition (version 2020.3)
    • IntelliJ at C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2020.3.3
    • 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 1.57.1)
    • VS Code at C:\Users\danya\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.23.0

[√] Connected device (4 available)
    • sdk gphone x86 (mobile) • emulator-5554 • android-x86    • Android 11 (API 30) (emulator)
    • Windows (desktop)       • windows       • windows-x64    • Microsoft Windows [Version 10.0.19041.1052]
    • Chrome (web)            • chrome        • web-javascript • Google Chrome 91.0.4472.106
    • Edge (web)              • edge          • web-javascript • Microsoft Edge 91.0.864.41

! Doctor found issues in 1 category.
```

</details>

cc @darrenaustin