Finish 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 248 matching lines @@
 
   static NativeBehavior deserializeNativeBehavior(ObjectDecoder decoder) {
     SideEffects sideEffects =
         new SideEffects.fromFlags(decoder.getInt(SIDE_EFFECTS));
     NativeBehavior behavior = new NativeBehavior.internal(sideEffects);
 
     behavior.typesReturned
         .addAll(decoder.getTypes(DART_TYPES_RETURNED, isOptional: true));
     behavior.typesReturned.addAll(decoder
         .getElements(THIS_TYPES_RETURNED, isOptional: true)
-        .map((element) => element.thisType)
+        .map((dynamic element) => element.thisType)
         .toList());
     behavior.typesReturned.addAll(decoder
         .getStrings(SPECIAL_TYPES_RETURNED, isOptional: true)
         .map(SpecialType.fromName));
 
     behavior.typesInstantiated
         .addAll(decoder.getTypes(DART_TYPES_INSTANTIATED, isOptional: true));
     behavior.typesInstantiated.addAll(decoder
         .getElements(THIS_TYPES_INSTANTIATED, isOptional: true)
-        .map((element) => element.thisType)
+        .map((dynamic element) => element.thisType)
         .toList());
     behavior.typesInstantiated.addAll(decoder
         .getStrings(SPECIAL_TYPES_INSTANTIATED, isOptional: true)
         .map(SpecialType.fromName));
 
     behavior.codeTemplateText =
         decoder.getString(CODE_TEMPLATE, isOptional: true);
     if (behavior.codeTemplateText != null) {
       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;
   }
 }