Fix all strong mode warnings in protoc-plugin

benchmark/lib/benchmarks/has_strings.dart

Index: benchmark/lib/benchmarks/has_strings.dart
diff --git a/benchmark/lib/benchmarks/has_strings.dart b/benchmark/lib/benchmarks/has_strings.dart
index 89430938b53e7e893599ae05bb6b540a425b6896..1dc9cd4bc5bb4c4b6718beff0d8b4fd9c871ca66 100644
--- a/benchmark/lib/benchmarks/has_strings.dart
+++ b/benchmark/lib/benchmarks/has_strings.dart
@@ -124,7 +124,7 @@ class HasStringsBenchmark extends Benchmark {
   static const $id = BenchmarkID.HAS_STRINGS;
   static final $type = new BenchmarkType($id, $create);
 
-  static $create(Request r) {
+  static HasStringsBenchmark $create(Request r) {
     assert(r.params.hasMessageCount());
     var value = null;
     if (r.params.hasStringValue()) value = r.params.stringValue;