protobuf benchs: fix old --trust-type-annotations flag (#622)

diff --git a/protobuf/benchmarks/compile-js.sh b/protobuf/benchmarks/compile-js.sh
index 0a93550..a232416 100755
--- a/protobuf/benchmarks/compile-js.sh
+++ b/protobuf/benchmarks/compile-js.sh
@@ -10,7 +10,7 @@
 
 mkdir -p "${OUTPUT_DIR}"
 
-dart2js --trust-type-annotations        \
+dart2js --omit-implicit-checks          \
         --trust-primitives              \
         -o "${OUTPUT_DIR}"/benchmark.js \
         "${SCRIPT_DIR}"/benchmark_js.dart