implement end-to-end dart unit test

PiperOrigin-RevId: 495384181
diff --git a/upbc/upbc_so.c b/upbc/upbc_so.c
index ac7809d..ac109e0 100644
--- a/upbc/upbc_so.c
+++ b/upbc/upbc_so.c
@@ -30,6 +30,7 @@
 #define UPB_BUILD_API
 
 #include "upb/mem/arena.h"
+#include "upb/message/accessors.h"
 #include "upb/message/message.h"
 #include "upb/mini_table/decode.h"
 #include "upbc/upbdev.h"