docs: public synchronized final void setClassShutter(ClassShutter shutter)
      {
          if (sealed) onSealedMutation();
          if (shutter == null) throw new IllegalArgumentException();
          if (hasClassShutter) {
              throw new SecurityException("Cannot overwrite existing " +
                                          "ClassShutter object");
          }
          classShutter = shutter;
          hasClassShutter = true;
      }