Fix all strong mode warnings in protoc-plugin

benchmark/lib/benchmarks/string_json.dart

Index: benchmark/lib/benchmarks/string_json.dart
diff --git a/benchmark/lib/benchmarks/string_json.dart b/benchmark/lib/benchmarks/string_json.dart
index 8e7c06672cbccd637ebb785b906b1ea3feb9b591..7d3cab24d73373c5aa7d22b4c8814fb1479321e8 100644
--- a/benchmark/lib/benchmarks/string_json.dart
+++ b/benchmark/lib/benchmarks/string_json.dart
@@ -86,7 +86,7 @@ class StringBenchmark extends Benchmark {
   static const $id = BenchmarkID.READ_STRING_FIELDS_JSON;
   static final $type = new BenchmarkType($id, $create);
 
-  static $create(Request r) {
+  static StringBenchmark $create(Request r) {
     assert(r.params.hasStringFieldCount());
     assert(r.params.hasMessageCount());
     assert(r.params.hasStringSize());