It would be awesome to have a table that shows all the ES6 features available by version of node/io.js. This is the closest thing I'm aware of:
https://kangax.github.io/compat-table/es6/
This is the closest thing to an official Node doc that I'm aware of:
For io.js, there is this page (although not quite up-to-date for 2.0.0+):
You can enter the following at the command line to see ES6 features that require a flag to be enabled:
node --v8-options | grep "harmony"
I'm a member of the io.js Evangelism Working Group, and we've blogged about the ES6 features available in io.js 2.0.0+ here:
Hope this helps,
Tony