Repository: flutter
Issue: `setSystemUIOverlayStyle().statusBarBrightness` & `setSystemUIOverlayStyle().statusBarIconBrightness` does not work properly on iOS
Labels: c: regression,platform-ios,framework,f: material design,a: layout,has reproducible steps,found in release: 2.1,found in release: 2.5,found in release: 2.6,team-design,triaged-design

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## Steps to Reproduce

<!-- You must include full steps to reproduce so that we can reproduce the problem. -->

1. Run `flutter create --sample=services.SystemChrome.setSystemUIOverlayStyle.2 mysample`.

**Expected results:** <!-- what did you want to see? -->
System status bar color change.

**Actual results:** <!-- what did you see? -->
Status bar color does not change on iOS devices.

<details>
  <summary>Logs</summary>

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[bug_log.txt](https://github.com/flutter/flutter/files/6122367/bug_log.txt)

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     If there are any analysis errors, try resolving them before filing this issue.
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```
Analyzing mysample...                                                   
No issues found! (ran in 1.9s)
```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
[✓] Flutter (Channel stable, 2.0.1, on macOS 11.2.2 20D80 darwin-x64, locale en-GB)
    • Flutter version 2.0.1 at /Users/develop/SDKs/flutter
    • Framework revision c5a4b4029c (7 days ago), 2021-03-04 09:47:48 -0800
    • Engine revision 40441def69
    • Dart version 2.12.0

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
    • Android SDK at /Users/develop/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 12.4, 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 1.54.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.20.0

[✓] Connected device (3 available)
    • sdk gphone x86 arm (mobile)       • emulator-5554                            • android-x86 • Android 11 (API 30) (emulator)
    • iPad 2018 Develop (3728) (mobile) • fe300c6fc96ac916a04068545991dce4733fcea4 • ios         • iOS 14.4.1
    • iPhone 12 Pro Max (mobile)        • 3F2E6288-280C-45EF-BCA4-1236F663267E     • ios         • com.apple.CoreSimulator.SimRuntime.iOS-14-4 (simulator)

! Doctor found issues in 1 category.
```

</details>