code: boolean addExtendedInterface(JSTypeExpression type) {
      lazyInitInfo();
      if (info.extendedInterfaces == null) {
        info.extendedInterfaces = new ArrayList<>(2);
      }
      if (info.extendedInterfaces.contains(type)) {
        return false;
      }
      info.extendedInterfaces.add(type);
      return true;
    }
docs: Adds an extended interface (for interface only).
  Returns whether the type was added.
  if the type was already present in the list, it won't get added again.