fix(package_info_plus): support AGP 9 / built-in Kotlin

[manuel-goester-n41]

Summary

AGP 9 removes support for applying the Kotlin Gradle Plugin (KGP). package_info_plus's Android module applies kotlin-android directly, so apps that depend on it fail to build under AGP 9, and Flutter 3.44+ already emits a warning:

WARNING: Your app uses the following plugins that apply Kotlin Gradle Plugin (KGP): … package_info_plus
Future versions of Flutter will fail to build if your app uses plugins that apply KGP.

This migrates package_info_plus to Flutter's built-in Kotlin in a backwards-compatible way. Closes #3881.

Changes

• Conditional KGP application — apply kotlin-android only when built-in Kotlin is not active, i.e. on AGP < 9 or when android.builtInKotlin=false (the Flutter transition default, which Flutter still sets to false even on AGP 9). This keeps the plugin building on both the legacy-KGP and built-in-Kotlin paths.
• compilerOptions DSL — replace the deprecated kotlinOptions { jvmTarget } block with kotlin { compilerOptions { jvmTarget = … } }, which works under both KGP and built-in Kotlin.
• Example app — migrated to built-in Kotlin (kotlin-android removed, compilerOptions DSL).

No change to the supported SDK range, compileSdk (already flutter.compileSdkVersion), the Kotlin version (already 2.2.0 — sufficient for built-in Kotlin on AGP 9), or runtime behavior. Build-configuration only; no public API change.

Verification

Built the example app with Flutter 3.44.2 in each relevant configuration:

Toolchain	android.builtInKotlin	Result
AGP 8.12.1 / Gradle 8.13	false	✅ builds; plugin applies KGP (legacy path)
AGP 9.0.1 / Gradle 9.1.0	false	✅ builds; KGP applied via Flutter's transition shim
AGP 9.0.1 / Gradle 9.1.0	true	✅ package_info_plus is KGP-free and configures cleanly under built-in Kotlin

Under built-in Kotlin the plugin no longer appears in Flutter's KGP warning.

Notes

• The guard reads android.builtInKotlin, so it automatically tracks whatever Kotlin mode the host app is in — no action required from existing consumers.
• Per the repo's Melos/conventional-commit workflow, no CHANGELOG.md edit is included; the changelog entry is derived from the fix(package_info_plus): … commit.

[vbuberen]

Thanks for your contribution, but I will handle this topic myself as there will be conflicting changes after merging #3858

[mdmahendri]

Thanks for your contribution, but I will handle this topic myself as there will be conflicting changes after merging #3858

@vbuberen Just following up on this. Do you plan to open PR for the built-in Kotlin compatibility changes, or would you like me to go with follow-up PR with the one already merged?

Also, regarding the addition of:

def builtInKotlin = providers.gradleProperty('android.builtInKotlin').map { it.toBoolean() }.orElse(agpMajor >= 9).get()

I was looking into whether that extra logic is actually necessary. It turns out the Flutter team didn't use that workaround in their example app migration (flutter/packages#11806).

This is because the new Flutter already detects a missing KGP declaration and automatically handles the injection under the hood via detectApplyingKotlinGradlePlugin (introduced in flutter/flutter#184227):

if (!hasKgpPlugin) {
    try {
        pluginManager.apply("kotlin-android")
    } ...
}

Given that the SDK handles the fallback automatically, we should be perfectly safe without manually parsing the android.builtInKotlin flag ourselves.

I tested by copying the code from this guide here https://docs.flutter.dev/release/breaking-changes/migrate-to-built-in-kotlin/for-plugin-authors#supporting-flutter-versions-earlier-than-3-44 using local environment and the KGP warning disappear.

[vbuberen]

Do you plan to open PR for the built-in Kotlin compatibility changes, or would you like me to go with follow-up PR with the one already merged?

I will open PRs myself. You already did enough of good contributions recently. And thanks a lot for such detailed descriptions - it is helpful and saves a lot of time.

[mdmahendri]

Great, thanks too! I will continue working on removing the Gradle warnings across the remaining packages then.

[evanrich]

this might still be broken. I just tried adding firebase_analytics to my app, and there's a warning from share_plus and package_info_plus about kotlin:

[mdmahendri]

I just did a quick test using the latest releases of package_info_plus 10.2.0 and share_plus 13.2.0. Both worked perfectly; running flutter build appbundle did not produce any warnings from either package.

Maybe you can open a new issue with step-by-step instructions using the provided example app. This will help demonstrate reproducibility and ensure the issue isn't isolated to your specific project configuration.

[vbuberen]

@evanrich The change follows the official documentation to support apps pre-3.44.x Flutter and I tend to trust the documentation more than whatever LLM threw out on the screenshot. As suggested by another person above - open a proper bug report with steps to reproduce if there is any actual bug.