docs: public static Ref callRef(Callable function, Scriptable thisObj,
                                Object[] args, Context cx)
      {
          if (function instanceof RefCallable) {
              RefCallable rfunction = (RefCallable)function;
              Ref ref = rfunction.refCall(cx, thisObj, args);
              if (ref == null) {
                  throw new IllegalStateException(rfunction.getClass().getName()+".refCall() returned null");
              }
              return ref;
          }
          // No runtime support for now
          String msg = getMessage1("msg.no.ref.from.function",
                                   toString(function));
          throw constructError("ReferenceError", msg);
      }