Hello, do you plan ES6 support?
If you will apply code transformer - I can hook into it with BabelJS

ES6 support is natively implemented in JDK9: https://bugs.openjdk.java.net/browse/JDK-8066046

You could try to install that version of Java, and see if it works.

I'm on the list of notifications for OpenJDK issues. Based on what I've seen, I don't think 1.9 will be used in general production for a while. It's just not mature enough. Most people are still considering migration from 1.7 to 1.8 or learning the nuances of 1.8. So my vote for now would be to hold on support for 1.9. It's just not necessary to jump into that pool of dirty water yet.

FYI : v1.7 is dead. v1.9 isn't here yet. So I'm building with JDK 1.8.141+
I'm also transitioning the code to be Nashorn-specific, rather than getting a generic "javascript" engine.

You can code with ES6 using this: https://blog.magikcraft.io/how-to-mod-minecraft-without-java-f076ddaec01c

Including executing ES6/TypeScript at the server console.