Flutter: Create a flutter project with Kotlin
I think the kotlin files can only be used in the platform-specific plugins. E.g. you have platform-specific implementation for both iOS and android and you can use kotlin for the Android part.The Flutter SDK is used directly with Dart.
Open your Android module in android so you can add your kotlin files
Flutter does not support Kotlin and it looks it will not allow Flutter to be written in Kotlin language. The biggest advantage of using Dart language is the management of the object. It has a great garbage collector. Don't you think google developers didn't think about that before? Of course, they did. But Dart is chosen as the one language of Flutter.