All strong mode cleaning of dart2js.

pkg/compiler/lib/src/js_backend/backend_serialization.dart

 // Copyright (c) 2016, 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 js_backend.serialization;
 
 import '../common/backend_api.dart' show BackendSerialization;
 import '../elements/elements.dart';
 import '../elements/resolution_types.dart';
 import '../elements/types.dart';
@@ skipping 89 matching lines @@
     NativeBehavior nativeFieldStoreBehavior =
         nativeData.nativeFieldStoreBehavior[element];
     if (nativeFieldStoreBehavior != null) {
       NativeBehaviorSerialization.serializeNativeBehavior(
           nativeFieldStoreBehavior,
           getEncoder().createObject(NATIVE_FIELD_STORE_BEHAVIOR));
     }
   }
 
   @override
-  void onData(NativeBehavior behavior, ObjectEncoder encoder) {
+  void onData(covariant NativeBehavior behavior, ObjectEncoder encoder) {
     NativeBehaviorSerialization.serializeNativeBehavior(behavior, encoder);
   }
 }
 
 class JavaScriptBackendDeserializer implements DeserializerPlugin {
   final JavaScriptBackend _backend;
 
   JavaScriptBackendDeserializer(this._backend);
 
   NativeBasicDataBuilderImpl get nativeBasicData =>
@@ skipping 167 matching lines @@
       behavior.codeTemplate = js.js.parseForeignJS(behavior.codeTemplateText);
     }
 
     behavior.throwBehavior =
         decoder.getEnum(THROW_BEHAVIOR, NativeThrowBehavior.values);
     behavior.isAllocation = decoder.getBool(IS_ALLOCATION);
     behavior.useGvn = decoder.getBool(USE_GVN);
     return behavior;
   }
 }