Proposal: `flutter config` flags can be specified in an app's `pubspec.yaml`

[matanlurey]

Today, Flutter has a globally-scoped¹ list of enabled feature flags, utilized by flutter config:

flutter config --enable-swift-package-manager

Swift Package Manager work has needed package-by-package opt-in/out support, implementing a 1-off:

flutter/packages/flutter_tools/test/general.shard/flutter_manifest_test.dart

Line 1412 in b73450f

disable-swift-package-manager: true

It's come up with discussions with various teams (I certainly felt this with explicit-package-dependencies) that there is a good user (end-user) and developer (contributor to Flutter) need to be able to enable or disable these feature flags on a per-project basis:

flutter:
  # Could also be called "flags", just using existing terminology for now.
  config:
    swift-package-manager: true
    explicit-package-dependencies: false

I'd like to propose the following:

1. We generalize the work done for disable-swift-package-manager (see above) or
2. We implement something with similar capabilities

My thoughts on the V1 specification:

A feature flag can be used to conditionally allow or disallow some set of functionality in the Flutter tool, which in turn could be used to inform how _other_ tools (such as Gradle, Xcode) or runtimes (such as the Flutter framework, engine, Impeller) behave or what capabilities they support.

Feature flags are derived from one of two locations:

1. The currently executing app (where a `flutter` command is being run)'s `pubspec.yaml` _or_
2. Globally, as specified by `flutter config --enable-x` (or `flutter config --disable-x`)

Feature flags that are defined in an app's `pubspec.yaml` take precedence. 

Feature flags only support simple boolean (`true` or `false`) values, which default based on the
current channel being used (`master`, `beta`, or stable`), which matches the current implementation
done in [`flutter_tools/lib/src/features.dart`][1].

[1]: https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/features.dart#L51-L55

/cc @Piinks @justinmc @jtmcdole

Footnotes

1. Well, to the %HOME directory. ↩

[dbrpci]

Our team uses fvm to manage flutter versions for our various projects. Today I was looking for a way to disable desktop platform targets by project, and it seems this proposal would be necessary to achieve that.

Currently the config file is global, potentially causing issues as we switch between projects (and other issues unrelated to platform targets), as that file won't be tracked in our git repos by project:

~/.config/flutter/settings

{
  "jdk-dir": "/opt/homebrew/opt/openjdk@17/",
  "enable-macos-desktop": false,
  "enable-windows-desktop": false,
  "enable-linux-desktop": false
}

If there's already a better way to achieve this, someone please let me know, otherwise I'm eager to see this proposal implemented. Thanks!

[github-actions]

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