I heard the Android data binding compiler was written in Kotlin. At KotlinConf, they announced that Google had dropped Java altogether for writing Lint rules because of Kotlin's much greater capabilities for handling deprecated code and IDE quick fixes.
Wasn't the linter thing just about moving the lint rules (for checking proper Android API usage) to a language-independent format so that they could be shared by Java and Kotlin? Previously they only worked for Java code.
You can watch the KotlinConf session video by Tor Norbye. It was what you said and more. To clarify, I meant they dropped Java internally for their own lint rules, not that they dropped support.
I know of one app for sure, but I don't know if I can tell :)
Director of project management, Android, said of Kotlin last July: "we're already using it ourselves to write code in Android Studio which gives us more ideas for making our support even better."
https://adtmag.com/articles/2017/07/20/android-ama.aspx