code: public RepeatedFieldBuilder<MType, BType, IType> addMessage(
        MType message) {
      if (message == null) {
        throw new NullPointerException();
      }
      ensureMutableMessageList();
      messages.add(message);
      if (builders != null) {
        builders.add(null);
      }
      onChanged();
      incrementModCounts();
      return this;
    }
docs: Appends the specified element to the end of this list.
  
  @param message the message to add
  @return the builder