Make ClosedWorld a `forTesting` property of ResolutionWorldBuilder.

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 80 matching lines @@
     await compilerDeserialized.run(entryPoint);
     compilerDeserialized.closeResolution();
     return compilerDeserialized;
   });
 
   return measure(title, 'check models', () async {
     checkAllImpacts(compilerNormal, compilerDeserialized, verbose: verbose);
     checkResolutionEnqueuers(compilerNormal.enqueuer.resolution,
         compilerDeserialized.enqueuer.resolution,
         verbose: verbose);
-    checkClosedWorlds(
-        compilerNormal.closedWorld, compilerDeserialized.closedWorld,
+    checkClosedWorlds(compilerNormal.resolverWorld.closedWorldForTesting,
+        compilerDeserialized.resolverWorld.closedWorldForTesting,
         verbose: verbose);
     checkBackendInfo(compilerNormal, compilerDeserialized, verbose: verbose);
   });
 }
 
 void checkResolutionEnqueuers(
     ResolutionEnqueuer enqueuer1, ResolutionEnqueuer enqueuer2,
     {bool typeEquivalence(DartType a, DartType b): areTypesEquivalent,
     bool elementFilter(Element element),
     bool verbose: false}) {
@@ skipping 321 matching lines @@
           a, b, (a, b) => areInstancesEquivalent(a, b, typeEquivalence)));
   return true;
 }
 
 bool areInstancesEquivalent(Instance instance1, Instance instance2,
     bool typeEquivalence(DartType a, DartType b)) {
   return typeEquivalence(instance1.type, instance2.type) &&
       instance1.kind == instance2.kind &&
       instance1.isRedirection == instance2.isRedirection;
 }