Actually I am thinking about renaming the script language to something like HiseScript, since there are many language differences to traditional Javascript (reg, namespace, include, inline function, local just to name the most important ones). On the other hand it's a valid superset of Javascript (so almost every Javascript construct should work in the HISE implementation so it's not "Yet Another Script Language".
About the classes thing, most encapsulation needs can be satisfied with the namespace concept that David sketched above, however if you are looking for any kind of inheritance or even polymorphism you're out of luck (to be honest, I don't know ECMA 6 well enough to know what's going on there).
Could you describe what kind of thing you want to achieve using classes? I am positive there's a solution in the current HISE implementation.