Access BackendUsage through ClosedWorld

tests/compiler/dart2js/serialization/model_test_helper.dart

Index: tests/compiler/dart2js/serialization/model_test_helper.dart
diff --git a/tests/compiler/dart2js/serialization/model_test_helper.dart b/tests/compiler/dart2js/serialization/model_test_helper.dart
index 3441b56b5c632fea3f505db0e441b3f92a1763a7..133e525440411612dd37e068abe4e3e1750ee71d 100644
--- a/tests/compiler/dart2js/serialization/model_test_helper.dart
+++ b/tests/compiler/dart2js/serialization/model_test_helper.dart
@@ -100,15 +100,17 @@ Future checkModels(Uri entryPoint,
 
   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);