All strong mode cleaning of dart2js.

tests/compiler/dart2js/serialization/members_test.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 dart2js.serialization.class_members_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/common/names.dart';
 import 'package:compiler/src/commandline_options.dart';
@@ skipping 39 matching lines @@
 }
 
 void checkAllMembers(Compiler compiler1, Compiler compiler2,
     {bool verbose: false}) {
   checkLoadedLibraryMembers(compiler1, compiler2,
       (Element member1) => member1 is ClassElement, checkMembers,
       verbose: verbose);
 }
 
 /// Check equivalence of members of [class1] and [class2].
-void checkMembers(Compiler compiler1, ClassMemberMixin class1,
-    Compiler compiler2, ClassMemberMixin class2,
+void checkMembers(
+    Compiler compiler1, Element _class1, Compiler compiler2, Element _class2,
     {bool verbose: false}) {
+  ClassMemberMixin class1 = _class1;
+  ClassMemberMixin class2 = _class2;
   if (verbose) {
     print('Checking $class1 vs $class2');
   }
   MembersCreator.computeAllClassMembers(compiler1.resolution, class1);
   MembersCreator.computeAllClassMembers(compiler2.resolution, class2);
 
   check(
       class1,
       class2,
       'interfaceMemberAreClassMembers',
@@ skipping 68 matching lines @@
       areTypesEquivalent);
   check(class1, class2, '$property.functionType', member1.functionType,
       member2.functionType, areTypesEquivalent);
   check(
       class1, class2, '$property.isGetter', member1.isGetter, member2.isGetter);
   check(
       class1, class2, '$property.isSetter', member1.isSetter, member2.isSetter);
   check(
       class1, class2, '$property.isMethod', member1.isMethod, member2.isMethod);
 }