The C API does not allow symbols as property names, this effects (principally) JSObjectGetProperty/SetProperty which still take JSStringRef, but we also need to update the class API callbacks. In addition to the accessors, we also need a way to create symbols without resorting to JS. Im not sure what such an API would look like, as we've already taken the obvious names -- we could determine string vs. non-string by checking the property name pointer against the GC heap, but that has interesting hazards, and i'm not sure it would be well received during api review :-/
It looks like there is an implementation on: https://bugs.webkit.org/show_bug.cgi?id=175836 I'll dup. *** This bug has been marked as a duplicate of bug 175836 ***