Repository: flutter
Issue: CupertinoAlertDialog inconsistent dividers across IOS and Android
Labels: framework,f: cupertino,a: quality,has reproducible steps,P2,found in release: 3.3,found in release: 3.7,team-design,triaged-design

## Steps to Reproduce

```
showDialog(
  context: context,
  builder: (BuildContext context) {
    return CupertinoAlertDialog(
      title: Text('title'),
      actions: [
        CupertinoDialogAction(child: Text('button')),
        CupertinoDialogAction(child: Text('button')),
      ],
    );
  },
);

showDialog(
  context: context,
  builder: (BuildContext context) {
    return CupertinoActionSheet(
      title: Text('title'),
      actions: [
        CupertinoDialogAction(child: Text('button')),
        CupertinoDialogAction(child: Text('button')),
      ],
    );
  },
);
```

**Expected results:** Dividers should look the same. They should be similar brightness as in the example image used here: [Cupertino (iOS-style) widgets](https://flutter.dev/docs/development/ui/widgets/cupertino)

<img src="https://flutter.dev/images/widget-catalog/cupertino-alert-dialog.png" width="300" />

**Actual results:** On iOS CupertinoAlertDialog divider is darker and thicker. In overall the divider is darker than it should be.

|                   Platform                   |                   CupertinoAlertDialog                   |                 CupertinoActionSheet                 |
|:---------------------------------------:|:---------------------------------------:|:---------------------------------------:|
| iOS | <img src="https://i.imgur.com/TErLjZA.jpg" width="300" /> | <img src="https://i.imgur.com/o5zAGjG.jpg" width="300" /> |
| Android | <img src="https://i.imgur.com/twJT7cQ.png" width="300" /> | <img src="https://i.imgur.com/SmC4kTn.png" width="300" /> |

<details>
  <summary>flutter doctor -v</summary>

```
[✓] Flutter (Channel stable, v1.17.5, on Mac OS X 10.15.5 19F101, locale en-LV)
    • Flutter version 1.17.5 at /Users/martinsandersons/Development/Flutter
    • Framework revision 8af6b2f038 (9 days ago), 2020-06-30 12:53:55 -0700
    • Engine revision ee76268252
    • Dart version 2.8.4
 
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at /Users/martinsandersons/Library/Android/sdk
    • Platform android-29, build-tools 29.0.2
    • Java binary at: /Users/martinsandersons/Library/Application Support/JetBrains/Toolbox/apps/AndroidStudio/ch-0/193.6514223/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)
    • 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 1.9.3

[✓] VS Code (version 1.46.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.12.1

[✓] Connected device (2 available)
    • SM G920F            • android-arm64 • Android 7.0 (API 24)
    • Martins’s iPhone SE • ios           • iOS 13.5.1

```

</details>