Do you plan to add a cross-platform assertion library?

What exactly do you mean? For testing, I have made a virtual view implementation to make unit testing views easy. As for mocking, like, mocking a server, I have a trick up my sleeve - instead of mocking the server, use an instance of the actual server. They are all written in Kotlin after all! Granted, I've left another system in place such that you could manually mock the server.

Something like http://joel-costigliola.github.io/assertj/

I'll think about it. If I do, it will be a fairly minimalistic extension to Kotlin's testing, as my goals with my libraries are to add the minimum number of declarations to make a task convenient and maximize the use of things in the standard library.

That's a nice idea!