Align semantics of getDefaultConstructor()

tests/compiler/dart2js/serialization/equivalence_test.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 dart2js.serialization_test;
 
 import 'dart:io';
 import '../memory_compiler.dart';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/commandline_options.dart';
@@ skipping 462 matching lines @@
 
     List<ConstructorElement> getConstructors(ClassElement cls) {
       return cls.implementation.constructors.map((c) => c.declaration).toList();
     }
 
     checkElementLists(
         element1, element2, 'constructors',
         getConstructors(element1),
         getConstructors(element2));
 
+    checkElementIdentities(element1, element2, 'defaultConstructor',
+        element1.lookupDefaultConstructor(),
+        element2.lookupDefaultConstructor());
+
     visitMembers(element1, element2);
 
     ClassElement superclass1 = element1.superclass;
     ClassElement superclass2 = element2.superclass;
     while (superclass1 != null && superclass1.isMixinApplication) {
       checkElementProperties(element1, element2, 'supermixin',
           superclass1, superclass2);
       superclass1 = superclass1.superclass;
       superclass2 = superclass2.superclass;
     }
@@ skipping 267 matching lines @@
   void visitPrefixElement(PrefixElement element1, PrefixElement element2) {
     check(
         element1, element2, 'isDeferred',
         element1.isDeferred, element2.isDeferred);
     checkElementIdentities(
         element1, element2, 'importedLibrary',
         element1.deferredImport, element2.deferredImport);
     // TODO(johnniwinther): Check members.
   }
 }