Fix all strong mode warnings in protoc-plugin

benchmark/lib/benchmarks/int64_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.int64_json;
 
 import 'package:fixnum/fixnum.dart' show Int64;
 
 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/int64grid.pb.dart' as pb;
 
 /// A benchmark that deserializes a grid of int fields.
 class Int64Benchmark extends Benchmark {
   final int width;
   final int height;
   String json;
   int lastFieldTag;
 
   Int64Benchmark(this.width, this.height) : super($id);
@@ skipping 51 matching lines @@
 
   @override
   measureSample(Sample s) => int64ReadsPerMillisecond(s);
 
   @override
   get measureSampleUnits => "int64 reads/ms";
 
   static const $id = BenchmarkID.READ_INT64_FIELDS_JSON;
   static final $type = new BenchmarkType($id, $create);
 
-  static $create(Request r) {
+  static Int64Benchmark $create(Request r) {
     assert(r.params.hasInt64FieldCount());
     assert(r.params.hasMessageCount());
     return new Int64Benchmark(r.params.int64FieldCount, r.params.messageCount);
   }
 }