Did anybody tried to use closure to remove unused code (including kotlin.js)?

You mean google closure compiler? It does not support ES5-style properties (Object.defineProperty) which is widely used in kotlin.js, hence it's unusable with Kotlin.

@konsoletyper Do you know other tools that could do the job? I currently use require.js to define my whole application and would like to have for production a single smallest js file.

I don't know of any such tool

And is there any plans at JB to create such tool?

Yes, there are such plans