Repository: flutter
Issue: AnimatedCrossFade not animating shrink
Labels: framework,a: animation,has reproducible steps,P2,found in release: 3.10,found in release: 3.11,team-framework,triaged-framework

## Steps to Reproduce

Change container width

Supposed problematic code

https://github.com/flutter/flutter/blob/b8a2456737c9645e5f3d7210fba6267f7408486f/packages/flutter/lib/src/widgets/animated_cross_fade.dart#L221-L231

<details>
<summary>Code sample</summary>

```dart
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: Home(),
    );
  }
}

class Home extends StatefulWidget {
  Home({Key key}) : super(key: key);

  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  bool first = false;

  void _setState() {
    setState(() {
      first = !first;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            AnimatedCrossFade(
              crossFadeState: first
                ? CrossFadeState.showFirst
                : CrossFadeState.showSecond,
              duration: const Duration(milliseconds: 300),
              firstChild: Container(
                color: Colors.blue,
                width: 100.0,
                height: 50.0,
              ),
              secondChild: Container(
                color: Colors.red,
                width: 50.0,
                height: 50.0,
              ),
            ),
            MaterialButton(
              child: Text('press'),
              onPressed: _setState,
            )
          ],
        ),
      ),
    );
  }
}

```
</details>

**Expected results:** AnimatedCrossFade always change its size

**Actual results:** AnimatedCrossFade animates grow but never animates shrink

<details>
  <summary>Logs</summary>

```
n/a
```

```
Analyzing issues...
No issues found! (ran in 2.8s)
```

```
[√] Flutter (Channel master, 2.3.0-2.0.pre.219, on Microsoft Windows [Version 10.0.19041.985], locale ru-RU)
    • Flutter version 2.3.0-2.0.pre.219 at C:\dev\src\flutter
    • Upstream repository git@github.com:nt4f04und/flutter.git
    • Framework revision aa9b2d914c (17 hours ago), 2021-05-14 14:54:03 -0700
    • Engine revision 5c33fa9113
    • Dart version 2.14.0 (build 2.14.0-117.0.dev)

[√] Android toolchain - develop for Android devices (Android SDK version 30.0.2) 
    • Android SDK at C:\Users\danya\AppData\Local\Android\sdk
    • Platform android-30, build-tools 30.0.2
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java       
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)
    • All Android licenses accepted.

[√] 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 1.56.2)
    • VS Code at C:\Users\danya\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.22.0

[√] 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 10.0.19041.985]
    • Chrome (web)            • chrome        • web-javascript • Google Chrome 90.0.4430.212
    • Edge (web)              • edge          • web-javascript • Microsoft Edge 90.0.818.46

• No issues found!
```

</details>

<img width="40%" src="https://user-images.githubusercontent.com/39104740/118366467-0309b380-b5a9-11eb-9356-a8840f16f605.gif">