Fix protoc_plugin run-tests make rule (#586)

diff --git a/protoc_plugin/Makefile b/protoc_plugin/Makefile
index 02718d8..a7556e3 100644
--- a/protoc_plugin/Makefile
+++ b/protoc_plugin/Makefile
@@ -96,7 +96,7 @@
 protos: $(PLUGIN_PATH) $(TEST_PROTO_LIBS)
 
 run-tests: protos
-	pub run test
+	dart run test
 
 clean:
 	rm -rf benchmark/lib/generated