code: public static boolean isProto2MessageClass(VisitorState state, Type type) {
      checkNotNull(type);
      return isAssignableTo(type, MESSAGE_TYPE, state)
          && !isAssignableTo(type, PROTOCOL_MESSAGE_TYPE, state);
    }
docs: Compile-time equivalent of {@code com.google.io.protocol.ProtocolSupport#isProto2MessageClass}.