Fix all strong mode warnings in protoc-plugin

benchmark/lib/benchmarks/int32_json.dart

Index: benchmark/lib/benchmarks/int32_json.dart
diff --git a/benchmark/lib/benchmarks/int32_json.dart b/benchmark/lib/benchmarks/int32_json.dart
index 1fb5850dbb4d0a36a554d6d34c52831f0b46a218..414ce4143c76e988637f9d2937cce46872a0b77d 100644
--- a/benchmark/lib/benchmarks/int32_json.dart
+++ b/benchmark/lib/benchmarks/int32_json.dart
@@ -5,7 +5,8 @@
 library protoc.benchmark.int32_json;
 
 import '../benchmark.dart';
-import '../generated/benchmark.pb.dart' show BenchmarkID, Request, Params, Sample;
+import '../generated/benchmark.pb.dart'
+    show BenchmarkID, Request, Params, Sample;
 import '../generated/int32grid.pb.dart' as pb;
 
 /// A benchmark that deserializes a grid of int fields.
@@ -77,7 +78,7 @@ class Int32Benchmark extends Benchmark {
   static const $id = BenchmarkID.READ_INT32_FIELDS_JSON;
   static final $type = new BenchmarkType($id, $create);
 
-  static $create(Request r) {
+  static Int32Benchmark $create(Request r) {
     assert(r.params.hasInt32FieldCount());
     assert(r.params.hasMessageCount());
     return new Int32Benchmark(r.params.int32FieldCount, r.params.messageCount);