I think they'd need an editor update first. 

Actually, I believe that you are able to use ES6 in the HTML environment if you add the type attribute to the script tag. So,

<script type>
    // Use ES6 syntax in here
</script>

The type attribute defaults to 'application/javascript' (ES6 JavaScript) but can be changed to make the script a GLSL shader.

Additionally, if you want to use let and const, you can add the comment // jshint esnext: true in the PJS environment to enable them. This doesn't add all of ES6 syntax though.

Is this what you were looking for?

Correction, you are able to use limited ES6. Because of KA's "live" update, some features such as top level const. In addition, the PJS environment can't use ES6 at all, to my knowledge. That being said, I believe the main purpose of the request is to request courses. 

Ummmmm to be honest, I'm still really new ... So I barely understand any of what you guys are talking about. I just dumped Khan and moved to FreeCodeCamp instead.

This is really late, but you can use ES6 syntax in PJS if you put the comment //jshint esnext:true at the top.

The PJS "esnext" is fake. It just converts the "let" and "const" to "var". Every other ES6 feature doesn't work.