code: @SuppressWarnings("unchecked")
    public MType getMessage() {
      if (message == null) {
        // If message is null, the invariant is that we must be have a builder.
        message = (MType) builder.buildPartial();
      }
      return message;
    }
docs: Get the message for the field. If the message is currently stored as a {@code Builder}, it is
  converted to a {@code Message} by calling {@link Message.Builder#buildPartial} on it. If no
  message has been set, returns the default instance of the message.
  
  @return the message for the field