Repository: flutter
Issue: VoiceOver doesn't recognise scroll using GestureDetector
Labels: platform-ios,framework,a: accessibility,f: gestures,has reproducible steps,P2,found in release: 2.2,found in release: 2.3,team-ios,triaged-ios

## Steps to Reproduce

1. Run `flutter create bug`.
2. Update the files as follows: 

**Replace the default child of Center() with a GestureDetector**

```dart
Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: GestureDetector(
          onVerticalDragEnd: (endDetails) {
            double velocity = endDetails.primaryVelocity;
            if        (velocity < 0) {
              print('GESTURE -- Swipe Up');
            } else if (velocity > 0) {
              print('GESTURE -- Swipe Down');
            }
          },
        )
      ),
```

3. Run the app (on physical device)
4. Turn on VoiceOver
5. Do a three finger swipe in the 'bug' app.

**Expected results:** 
Without VoiceOver activated, the app registers scroll action with a one finger swipe.
With VoicerOver activated, the app registers scroll actions with a three finger swipe.

[Three finger swipe is how users with VoiceOver activated scroll.](https://support.apple.com/guide/iphone/learn-voiceover-gestures-iph3e2e2281/ios)

**Actual results:**
As expected, without VoiceOver accessibility turned on, the scrolling using (one finger) swipe works. 
Unexpected, with VoiceOver on, the GestureDetector widget does not register a swipe action and users are not able to scroll using a three finger swipe.

<details>


```
flutter analyze
Analyzing voiceoverscrolltest...                                        

   info • The value of the field '_counter' isn't used • lib/main.dart:53:7 • unused_field

1 issue found. (ran in 3.8s)
```



```
flutter doctor -v
[✓] Flutter (Channel stable, 1.22.5, on Mac OS X 10.15.7 19H524 darwin-x64, locale en-NL)
    • Flutter version 1.22.5 at /Users/Mat/Documents/development/flutter
    • Framework revision 7891006299 (6 months ago), 2020-12-10 11:54:40 -0800
    • Engine revision ae90085a84
    • Dart version 2.10.4

[✗] Android toolchain - develop for Android devices
    ✗ Unable to locate Android SDK.
      Install Android Studio from: https://developer.android.com/studio/index.html
      On first launch it will assist you in installing the Android SDK components.
      (or visit https://flutter.dev/docs/get-started/install/macos#android-setup for detailed instructions).
      If the Android SDK has been installed to a custom location, set ANDROID_SDK_ROOT to that location.
      You may also want to add it to your PATH environment variable.

[✓] Xcode - develop for iOS and macOS (Xcode 12.3)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.3, Build version 12C33
    • CocoaPods version 1.10.1

[!] Android Studio (not installed)
    • Android Studio not found; download from https://developer.android.com/studio/index.html
      (or visit https://flutter.dev/docs/get-started/install/macos#android-setup for detailed instructions).

[✓] VS Code (version 1.56.2)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.22.0

[✓] Connected device (1 available)    
```

</details>