docs: public static Scriptable getClassPrototype(Scriptable scope,
                                                 String className)
      {
          scope = getTopLevelScope(scope);
          Object ctor = getProperty(scope, className);
          Object proto;
          if (ctor instanceof BaseFunction) {
              proto = ((BaseFunction)ctor).getPrototypeProperty();
          } else if (ctor instanceof Scriptable) {
              Scriptable ctorObj = (Scriptable)ctor;
              proto = ctorObj.get("prototype", ctorObj);
          } else {
              return null;
          }
          if (proto instanceof Scriptable) {
              return (Scriptable)proto;
          }
          return null;
      }