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