Repository: flutter
Issue: CupertinoPageRoute barrierColor is visible on a Scaffold beneath a transparent Widget
Labels: c: regression,framework,f: cupertino,f: routes,has reproducible steps,P2,found in release: 3.0,found in release: 3.1,team-design,triaged-design

## Steps to Reproduce
Please check [the DartPad here.](https://dartpad.dev/?id=95e0d2158d518965a8aebb817c79b50c)

## Notes: 
- I've determined that this issue is the Navigator object wrapping my `_page` property in the above DartPad.
- This exact code was rendering as expected on Flutter V2
- Upgraded to V3 and the bug is present.
- There is a comment on [this StackOverflow](https://stackoverflow.com/questions/72563512/flutter-why-is-navigator-changing-colour-of-scaffold-beneath-transparent-widget) question by someone who ran it on V2 and said it was fine.

**Note:** Bug result is same on iOS 15 and Android 12 physical devices
 
## To see the wrong/right render:

1. Load the above DartPad.
2. Toggle L.156 commented / uncommented 
3. Run
4. Observe that the render of the Scaffold blue colour is not consistent.

**Expected results:**

I expected: 
- there to be no discernible line or shading between the `BottomNavigationBar` and the `Scaffold` underneath
- the blue colour to be fully opaque, not somewhat "transparent" as it appears.

**Actual results:** 

- the blue colour of the underlying Scaffold is somewhat "transparent".


## Screenshots:

Bad:

<img width="776" alt="bad" src="https://user-images.githubusercontent.com/14280030/172940120-70c57086-1bd1-4923-8c4b-6f9c12eab155.png">

Good:

<img width="776" alt="good" src="https://user-images.githubusercontent.com/14280030/172940183-d65cd308-9970-4735-87fb-bdbb01847659.png">


<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
Davids-MacBook-Pro:moodment daveshirman$ flutter doctor -v
[✓] Flutter (Channel stable, 3.0.1, on macOS 11.6.6 20G624 darwin-x64, locale en-GB)
    • Flutter version 3.0.1 at /Users/daveshirman/fvm/versions/stable
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision fb57da5f94 (3 weeks ago), 2022-05-19 15:50:29 -0700
    • Engine revision caaafc5604
    • Dart version 2.17.1
    • DevTools version 2.12.2

[!] Android toolchain - develop for Android devices (Android SDK version 31.0.0)
    • Android SDK at /Users/daveshirman/Library/Android/sdk
    ✗ cmdline-tools component is missing
      Run `path/to/sdkmanager --install "cmdline-tools;latest"`
      See https://developer.android.com/studio/command-line for more details.
    ✗ Android license status unknown.
      Run `flutter doctor --android-licenses` to accept the SDK licenses.
      See https://flutter.dev/docs/get-started/install/macos#android-setup for more details.

[✓] Xcode - develop for iOS and macOS (Xcode 13.2.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • CocoaPods version 1.11.3

[✓] Android Studio (version 2021.1)
    • 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 11.0.11+0-b60-7590822)

[✓] VS Code (version 1.67.2)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.42.0

[✓] Connected device (1 available)
    • iPhone 13 mini (mobile) • FCD4F8E9-8E88-4C8C-B901-46D70A186315 • ios • com.apple.CoreSimulator.SimRuntime.iOS-15-2 (simulator)

[✓] HTTP Host Availability
    • All required HTTP hosts are available

! Doctor found issues in 1 category.
```