Fix all strong mode warnings in protoc-plugin

benchmark/lib/benchmarks/string_json.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library protoc.benchmark.string_json;
 
 import '../benchmark.dart';
 import '../generated/benchmark.pb.dart'
     show BenchmarkID, Request, Params, Sample;
 import '../generated/string_grid.pb.dart' as pb;
@@ skipping 68 matching lines @@
 
   @override
   measureSample(Sample s) => stringReadsPerMillisecond(s);
 
   @override
   get measureSampleUnits => "string reads/ms";
 
   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());
     return new StringBenchmark(
         r.params.stringFieldCount, r.params.messageCount, r.params.stringSize);
   }
 }