I wonder what the policy is for importing classes from java.* et al in projects intended not only for the JVM?
Take a look at this commit, which I believe is not correct: https://github.com/JetBrains/kotlin/commit/be7989af36197879774e55335216ca0fa4b6df5d
As you can see in the comment section below the commits, despite there being an import java.util.StringBuilder the resulting Javscript code
works just fine since there is an implementation of StringBuilder in the js-stdlib.