[ObjC] Add tests for proto3 optional behaviors.
diff --git a/objectivec/DevTools/compile_testing_protos.sh b/objectivec/DevTools/compile_testing_protos.sh
index dc1d6d2..d04c5c5 100755
--- a/objectivec/DevTools/compile_testing_protos.sh
+++ b/objectivec/DevTools/compile_testing_protos.sh
@@ -158,6 +158,7 @@
     --objc_out="${OUTPUT_DIR}/google/protobuf" \
     --proto_path=src/google/protobuf/          \
     --proto_path=src                           \
+    --experimental_allow_proto3_optional       \
     "$@"
 }