Access BackendUsage through ClosedWorld

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 82 matching lines @@
         resolutionInputs: resolutionInputs,
         options: [Flags.analyzeOnly]);
     compilerDeserialized.resolution.retainCachesForTesting = true;
     await compilerDeserialized.run(entryPoint);
     compilerDeserialized.closeResolution();
     return compilerDeserialized;
   });
 
   return measure(title, 'check models', () async {
     checkAllImpacts(compilerNormal, compilerDeserialized, verbose: verbose);
+    ClosedWorld closedWorld1 =
+        compilerNormal.resolutionWorldBuilder.closedWorldForTesting;
+    ClosedWorld closedWorld2 =
+        compilerNormal.resolutionWorldBuilder.closedWorldForTesting;
     checkResolutionEnqueuers(
-        compilerNormal.backend.backendUsage,
-        compilerDeserialized.backend.backendUsage,
+        closedWorld1.backendUsage,
+        closedWorld2.backendUsage,
         compilerNormal.enqueuer.resolution,
         compilerDeserialized.enqueuer.resolution,
         verbose: verbose);
-    checkClosedWorlds(
-        compilerNormal.resolutionWorldBuilder.closedWorldForTesting,
-        compilerDeserialized.resolutionWorldBuilder.closedWorldForTesting,
+    checkClosedWorlds(closedWorld1, closedWorld2,
         // Serialized native data include non-live members.
         allowExtra: true,
         verbose: verbose);
     checkBackendInfo(compilerNormal, compilerDeserialized, verbose: verbose);
   });
 }
 
 void checkResolutionEnqueuers(
     BackendUsage backendUsage1,
     BackendUsage backendUsage2,
@@ skipping 460 matching lines @@
 }
 
 bool areInstancesEquivalent(Instance instance1, Instance instance2,
     bool typeEquivalence(DartType a, DartType b)) {
   InterfaceType type1 = instance1.type;
   InterfaceType type2 = instance2.type;
   return typeEquivalence(type1, type2) &&
       instance1.kind == instance2.kind &&
       instance1.isRedirection == instance2.isRedirection;
 }