Reverts #132985
Initiated by: christopherfujino
This change reverts the following previous change:
Original Description:
Provides support for conditional bundling of assets through the existing --flavor option for flutter build and flutter run. Closes #21682. Resolves #136092

Change

Within the assets section pubspec.yaml, the user can now specify one or more flavors that an asset belongs to. Consider this example:

# pubspec.yaml
flutter:
  assets:
    - assets/normal-asset.png
    - path: assets/vanilla/ice-cream.png
      flavors: 
        - vanilla
    - path: assets/strawberry/ice-cream.png
      flavors:
        - strawberry

With this pubspec,

• flutter run --flavor vanilla will not include assets/strawberry/ice-cream.png in the build output.
• flutter run --flavor strawberry will not include assets/vanilla/ice-cream.png.
• flutter run will only include assets/normal-asset.png.

Open questions

• Should this be supported for all platforms, or should this change be limited to ones with documented --flavor support (Android, iOS, and (implicitly) MacOS)? This PR currently only enables this feature for officially supported platforms.

Design thoughts, what this PR does not do, etc.

This does not provide an automatic mapping/resolution of asset keys/paths to others based on flavor at runtime.

The implementation in this PR represents a simplest approach. Notably, it does not give Flutter the ability to dynamically choose an asset based on flavor using a single asset key. For example, one can't use Image.asset('config.json') to dynamically choose between different "flavors" of config.json (such as dev-flavor/config.json or prod-flavor/config.json). However, a user could always implement such a mechanism in their project or in a library by examining the flavor at runtime.

When multiple entries affect the same file and 1) at least one of these entries have a flavors list provided and 2) these lists are not equivalent, we always consider the manifest to be ambiguous and will throw a ToolExit.

assets:
  - assets/
  - path: assets/vanilla.png
    flavors: 
      - vanilla

assets:
  - path: assets/vanilla/
    flavors: 
      - vanilla
  - path: assets/vanilla/cherry.png
     flavor:
      - cherry

# Thinking towards the future where we might add glob/regex support and more conditions other than flavor:
assets:
  - path: assets/vanilla/**
    flavors:
      - vanilla
  - path: assets/**/ios/**
    platforms: 
       - ios

# Ambiguous in the case of assets like "assets/vanilla/ios/icon.svg" since we 
# don't know if flavor `vanilla` and platform `ios` should be combined using or-logic or and-logic.

This does not support Android's multidimensional flavors feature (in an intuitive way)

android {
    // ...

    flavorDimensions "mode", "api"

    productFlavors {
        free {
            dimension "mode"
            applicationIdSuffix ".free"
        }

        premium {
            dimension "mode"
            applicationIdSuffix ".premium"
        }

        minApi23 {
            dimension "api"
            versionNameSuffix "-minApi23"
        }

        minApi21 {
            dimension "api"
            versionNameSuffix "-minApi21"
        }
    }
}

assets:
  - assets/free/
    flavors:
      - freeMinApi21
      - freeMinApi23

See https://flutter.dev/go/flavor-specific-assets for the (outdated) design document.

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• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.