Firebase Cloud Messaging for Flutter

[TillCity]

The instructions here for implementing Firebase messaging in Flutter are very comprehensive and easy to understand. However, I got a little lost with the Optionally handle background messages section. Step 2 reads

Add an Application.java class to your app in the same directory as your MainActivity.java

All new Flutter projects use Kotlin, not Java, by default - AFAIK this has been so for at least several months now. I have a MainActivity.kt file, no MainActivity.java file. What do I do here? Convert the supplied Application.java to Application.kt line by line?