Extract ResolutionWorldBuilderBase from ElementResolutionWorldBuilder

tests/compiler/dart2js/serialization/model_test_helper.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_model_test;
 
 import 'dart:async';
 import 'dart:io';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
@@ skipping 433 matching lines @@
       'instantiationMap of\n   '
       '${info1.instantiationMap}\nvs ${info2.instantiationMap}',
       areElementsEquivalent,
       (a, b) => areSetsEquivalent(
           a, b, (a, b) => areInstancesEquivalent(a, b, typeEquivalence)));
   return true;
 }
 
 bool areInstancesEquivalent(Instance instance1, Instance instance2,
     bool typeEquivalence(ResolutionDartType a, ResolutionDartType b)) {
-  return typeEquivalence(instance1.type, instance2.type) &&
+  ResolutionInterfaceType type1 = instance1.type;
+  ResolutionInterfaceType type2 = instance2.type;
+  return typeEquivalence(type1, type2) &&
       instance1.kind == instance2.kind &&
       instance1.isRedirection == instance2.isRedirection;
 }