Repository: flutter
Issue: ListView.builder and SliverList with SliverChildBuilderDelegate: implicit scrolling doesn't work with iOS VoiceOver when children are tall
Labels: platform-ios,framework,a: accessibility,f: scrolling,has reproducible steps,P2,found in release: 2.0,team-ios,triaged-ios

<!-- Thank you for using Flutter!

     If you are looking for support, please check out our documentation
     or consider asking a question on Stack Overflow:
      * https://flutter.dev/
      * https://api.flutter.dev/
      * https://stackoverflow.com/questions/tagged/flutter?sort=frequent

     If you have found a bug or if our documentation doesn't have an answer
     to what you're looking for, then fill out the template below. Please read
     our guide to filing a bug first: https://flutter.dev/docs/resources/bug-reports
-->

## Steps to Reproduce

1. Run application on an iOS device (I tested on an iPhone 6s, iOS 14.4.1).

Using ListView:
```
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        appBar: AppBar(
          title: Text("ListView.builder Accessibility"),
        ),
        body: ListView.builder(
          itemBuilder: (BuildContext context, int index) {
            return Container(
              height: MediaQuery.of(context).size.height * 2,
              child: Text('$index'),
            );
          },
        ),
      ),
    );
  }
}
```

Or using SliverList:
```
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        appBar: AppBar(
          title: Text("ListView.builder Accessibility"),
        ),
        body: CustomScrollView(
          slivers: [
            SliverList(
              delegate: SliverChildBuilderDelegate(
                (BuildContext context, int index) {
                  return Container(
                    height: MediaQuery.of(context).size.height * 2,
                    child: Text('$index'),
                  );
                },
              ),
            ),
          ],
        ),
      ),
    );
  }
}
```

2. Enable VoiceOver.
3. Swipe left repeatedly to cycle accessibility focus through the list children.

**Expected results:** <!-- what did you want to see? -->

The list view should implicitly scroll as necessary, with VoiceOver focusing on the next child as it comes onto the screen. This works as expected on Android.

**Actual results:** <!-- what did you see? -->

The list view does not scroll and subsequent list children are not focused. 

<details>
<!--
     Run `flutter analyze` and attach any output of that command below.
     If there are any analysis errors, try resolving them before filing this issue.
-->

```
$ flutter analyze
Analyzing listview_builder_accessibility...                             
No issues found! (ran in 10.2s)
```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
$ flutter doctor -v
[✓] Flutter (Channel stable, 2.0.3, on Mac OS X 10.15.7 19H524 darwin-x64, locale en-US)
    • Flutter version 2.0.3 at /Users/jonathan.joelson/repos/flutter
    • Framework revision 4d7946a68d (8 days ago), 2021-03-18 17:24:33 -0700
    • Engine revision 3459eb2436
    • Dart version 2.12.2

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
    • Android SDK at /Users/jonathan.joelson/Library/Android/sdk
    • Platform android-30, build-tools 30.0.3
    • 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 12.4, Build version 12D4e
    • CocoaPods version 1.10.1

[✗] Chrome - develop for the web (Cannot find Chrome executable at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome)
    ! Cannot find Chrome. Try setting CHROME_EXECUTABLE to a Chrome executable.

[✓] 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 1.54.3)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.20.0

[✓] Connected device (3 available)
    • Pixel 4a (mobile)                  • 0B201JEC221560                           • android-arm64 • Android 11 (API 30)
    • Jonathan Joelson’s iPhone (mobile) • 5adff574a1e6e01ea07fbce2fbfd86e78dd84167 • ios           • iOS 14.4.1
    • iPhone 12 Pro (mobile)             • 6466B0CE-0126-49BE-886F-4376707E20C9     • ios           • com.apple.CoreSimulator.SimRuntime.iOS-14-4
      (simulator)

! Doctor found issues in 1 category.
```

</details>