Repository: flutter
Issue: Adding a `Widget` to `Scaffold.bottomSheet` doesn't animate its appearance
Labels: framework,a: animation,f: material design,has reproducible steps,found in release: 3.3,found in release: 3.7,team-design,triaged-design

While I've recently created #83668 and I think these issues may be related, but overall they are not **precisely** the same problem.

So, the unexpected behavior is that when adding a non-null `Widget` to `Scaffold.bottomSheet`, it doesn't animate (appearance animation) like it does when being dismissed.

## Steps to Reproduce

1a. Run the code below in any mobile simulator/emulator;
<details>
  <summary>Reproducible Error</summary>

```dart
import 'package:flutter/material.dart';

void main() => runApp(BottomSheetApp());

class BottomSheetApp extends StatefulWidget {
  @override
  _BottomSheetAppState createState() => _BottomSheetAppState();
}

class _BottomSheetAppState extends State<BottomSheetApp> {
  bool showsSheet = false;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: ElevatedButton(
            child: Text('Toggle sheet'),
            onPressed: () => setState(() => showsSheet = !showsSheet),
          ),
        ),
        bottomSheet: showsSheet ? Container(color: Colors.blue) : null,
      ),
    );
  }
}
```
</details>

1b. Emulate the same scenario using [this dartpad that I've created with the code above](https://dartpad.dev/6b57991e4c09a7116a033bca79542476?null_safety=true)
2. Press the `Toggle Sheet` button to make it appear (it doesn't animate);
3. [Optional] Press the `Toggle Sheet` button again to set it to `null`, thus making it disappear. You can see that the dismissal animation works as expected.

<details>
  <summary><pre>flutter doctor -v</pre></summary>

```
[✓] Flutter (Channel stable, 2.2.0, on macOS 11.2.1 20D74 darwin-x64, locale en-BR)
    • Flutter version 2.2.0 at /Users/guilhermematuella/flutter
    • Framework revision b22742018b (2 weeks ago), 2021-05-14 19:12:57 -0700
    • Engine revision a9d88a4d18
    • Dart version 2.13.0

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
    • Android SDK at /Users/guilhermematuella/Library/Android/sdk
    • Platform android-30, build-tools 30.0.2
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.2, Build version 12B45b
    • CocoaPods version 1.10.1

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 4.1)
    • Android Studio at /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 1.8.0_242-release-1644-b3-6915495)

[✓] VS Code (version 1.56.2)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.22.0

[✓] Connected device (3 available)
    • iPhone SE (2nd generation) (mobile) • 14A9935F-51BF-41D7-AE3B-94B23FBDDCD7 • ios            • com.apple.CoreSimulator.SimRuntime.iOS-14-2 (simulator)
    • macOS (desktop)                     • macos                                • darwin-x64     • macOS 11.2.1 20D74 darwin-x64
    • Chrome (web)                        • chrome                               • web-javascript • Google Chrome 90.0.4430.212

• No issues found!
```
</details>