code: public BType addBuilder(final MType message) {
      ensureMutableMessageList();
      ensureBuilders();
      final SingleFieldBuilder<MType, BType, IType> builder =
          newSingleFieldBuilder(message, this, isClean);
      messages.add(null);
      builders.add(builder);
      onChanged();
      incrementModCounts();
      return builder.getBuilder();
    }
docs: Appends a new builder to the end of this list and returns the builder.
  
  @param message the message to add which is the basis of the builder
  @return the new builder