code: public boolean usePrimaryMsg(long msgId, long fallbackId) {
      // Note: we need to make sure the fallback msg is actually present if we are going to fallback.
      // use getMsgParts() since if the bundle is a RenderOnlySoyMsgBundleImpl then this will be
      // allocation free.
      return !msgBundle.getMsgParts(msgId).isEmpty() || msgBundle.getMsgParts(fallbackId).isEmpty();
    }
docs: Returns {@code true} if the primary msg should be used instead of the fallback.