Repository: flutter
Issue: Stack widget does not clip children wrapped in Align
Labels: framework,has reproducible steps,P2,found in release: 3.3,found in release: 3.7,team-framework,triaged-framework

According to this documentation: [flutter.dev/go/clip-behavior](flutter.dev/go/clip-behavior), Stack is one of the few widgets that will clip it's children by default (as `{Clip clipBehavior = Clip.hardEdge}` is the default). However, even with a simple example (which I've pasted below) this is not the case for children wrapped in Align.

Here is an example **main.dart** that recreates the issue:
<details>
<summary>code sample</summary>

```dart
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) => MaterialApp(
        home: Scaffold(
          body: SafeArea(
            child: Row(
              children: [
                Expanded(
                  child: Container(
                    decoration: BoxDecoration(
                      border: Border.all(color: Colors.green),
                    ),
                    child: Stack(
                      children: [
                        Align(
                          alignment: Alignment(-0.98, 0.43),
                          child: Container(
                            width: 120,
                            height: 120,
                            decoration: BoxDecoration(
                              shape: BoxShape.circle,
                              color: Colors.black,
                            ),
                          ),
                        ),
                        Align(
                          alignment: Alignment(1.8, 1),
                          child: Container(
                            width: 120,
                            height: 120,
                            decoration: BoxDecoration(
                              shape: BoxShape.circle,
                              color: Colors.blue,
                            ),
                          ),
                        ),
                      ],
                    ),
                  ),
                ),
                Container(width: 100, height: 100, color: Colors.red),
              ],
            ),
          ),
        ),
      );
}
```
</details> 

**Expected results**: The child with `Alignment(1.8, 1)` extends beyond the bounds of the Stack and should be clipped.
**Actual results**: The child with `Alignment(1.8, 1)` is not clipped and is painted beyond the bounds of the Stack.

Output of `flutter analyze`:
```
Running "flutter pub get" in bug...                                408ms
Analyzing bug...                                                        
No issues found! (ran in 3.2s)
```

Output of `flutter doctor -v`:
```
[✓] Flutter (Channel beta, 1.24.0-10.2.pre, on Mac OS X 10.15.6 19G73 darwin-x64, locale en-US)
    • Flutter version 1.24.0-10.2.pre at /Users/agreaves/flutter
    • Framework revision 022b333a08 (2 weeks ago), 2020-11-18 11:35:09 -0800
    • Engine revision 07c1eed46b
    • Dart version 2.12.0 (build 2.12.0-29.10.beta)

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at /Users/agreaves/Library/Android/sdk
    • Platform android-29, build-tools 29.0.2
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 11.5)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 11.5, Build version 11E608c
    • CocoaPods version 1.9.1

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 3.5)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 40.2.2
    • Dart plugin version 191.8580
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)

[✓] VS Code (version 1.51.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.16.0

[✓] Connected device (3 available)
    • Android SDK built for x86 (mobile) • emulator-5554 • android-x86    • Android 10 (API 29) (emulator)
    • Web Server (web)                   • web-server    • web-javascript • Flutter Tools
    • Chrome (web)                       • chrome        • web-javascript • Google Chrome 86.0.4240.198

• No issues found!
```

Here's a snapshot of the result (it's basically just what I described):
<img src="https://user-images.githubusercontent.com/7757399/100950985-6d7adb80-34c2-11eb-91e1-e847e4550886.png" width ="300">