Affects the master channel.

#132985 landed a new feature, but it doesn't work for macOS.

We should fix this and also update the recently restored device lab tests to test this behavior.

Repro.

• Use this project, https://github.com/andrewkolos/_flavors_test_world/tree/main/flavor_specific_assets, which is a Flutter project set up with flavors and flavor-conditional assets (see pubspec.yaml for configuration).
• flutter run -d macos --flavor vanilla
• App should launch. The app will display vanilla as the flavor, but assets with the flavor of vanilla (see pubspec) won't be included in the asset bundle.