We add support for Built-in Kotlin when we have set the default to KGP to ensure the Flutter ecosystem can safely migrate. If app or plugin developers have not made the migration, we log relevant warnings and links to migration guides. The Flutter ecosystem must migrate to Built-in Kotlin in order to use AGP >= 9.

Implementation-wise, we are maintaining the legacy KGP as the default behavior. However, by adding internal support for Built-in Kotlin, we have effectively enabled AGP 9+ compatibility for the end user. So mechanically, we are supporting Built-in Kotlin but really this work was done to support the legacy KGP on AGP 9+ that way all migration states are supported.