Repository: flutter
Issue: Deep linking doesn't work on macOS, onGenerateRoute isn't called
Labels: engine,platform-mac,a: desktop,P2,team-macos,triaged-macos

## Steps to Reproduce

Example project: https://github.com/viktorbenei/flutter-github-api-example/tree/f24935cfdf9faa6d56b3062cd5620a40d1211ded
`macos/Runner/Info.plist` (https://github.com/viktorbenei/flutter-github-api-example/blob/f24935cfdf9faa6d56b3062cd5620a40d1211ded/macos/Runner/Info.plist#L21) includes 

```
<key>FlutterDeepLinkingEnabled</key>
<true/>
```

as well as

```
<key>CFBundleURLTypes</key>
	<array>
		<dict>
			<key>CFBundleTypeRole</key>
			<string>Editor</string>
			<key>CFBundleURLName</key>
			<string>com.example.flutterGithubApiExample</string>
			<key>CFBundleURLSchemes</key>
			<array>
				<string>flutter-github-api-example</string>
			</array>
		</dict>
	</array>
```

same as the iOS `Info.plist` (https://github.com/viktorbenei/flutter-github-api-example/blob/f24935cfdf9faa6d56b3062cd5620a40d1211ded/ios/Runner/Info.plist#L21)


**Expected results:** When the app is opened via a deep link (URL Scheme on iOS & macOS) `onGenerateRoute` (https://github.com/viktorbenei/flutter-github-api-example/blob/f24935cfdf9faa6d56b3062cd5620a40d1211ded/lib/main.dart#L41) should be called with the parameters passed via the link.

**Actual results:** Works as expected on iOS, while on macOS `onGenerateRoute` isn't called at all. The app is opened/shown by macOS when the `flutter-github-api-example://...` URL is called, but `onGenerateRoute` isn't invoked.

<details>
  <summary>Logs</summary>

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<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
flutter doctor -v

[✓] Flutter (Channel stable, 2.2.2, on macOS 11.4 20F71 darwin-x64, locale en-HU)
    • Flutter version 2.2.2 at /Users/viktorbenei/flutter
    • Framework revision d79295af24 (3 weeks ago), 2021-06-11 08:56:01 -0700
    • Engine revision 91c9fc8fe0
    • Dart version 2.13.3

[!] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
    • Android SDK at /Users/viktorbenei/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 11.0.8+10-b944.6916264)
    ✗ 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 at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.5.1, Build version 12E507
    • CocoaPods version 1.10.1

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 4.2)
    • 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 11.0.8+10-b944.6916264)

[✓] VS Code (version 1.57.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.24.0

[✓] Connected device (2 available)
    • macOS (desktop) • macos  • darwin-x64     • macOS 11.4 20F71 darwin-x64
    • Chrome (web)    • chrome • web-javascript • Google Chrome 91.0.4472.114

! Doctor found issues in 1 category.
```

</details>