Access BackendUsage through ClosedWorld

tests/compiler/dart2js/kernel/closed_world2_test.dart

Index: tests/compiler/dart2js/kernel/closed_world2_test.dart
diff --git a/tests/compiler/dart2js/kernel/closed_world2_test.dart b/tests/compiler/dart2js/kernel/closed_world2_test.dart
index 39b26e302486890d23ed46541cf41c382034b622..e161442dc78c5e682e8bb151d59e898c7a3918ba 100644
--- a/tests/compiler/dart2js/kernel/closed_world2_test.dart
+++ b/tests/compiler/dart2js/kernel/closed_world2_test.dart
@@ -142,8 +142,8 @@ Future<ResultKind> mainInternal(List<String> args,
   }
   Expect.isFalse(compiler1.compilationFailed);
   ResolutionEnqueuer enqueuer1 = compiler1.enqueuer.resolution;
-  BackendUsage backendUsage1 = compiler1.backend.backendUsage;
   ClosedWorld closedWorld1 = compiler1.resolutionWorldBuilder.closeWorld();
+  BackendUsage backendUsage1 = closedWorld1.backendUsage;
 
   Pair<Compiler, Compiler> compilers =
       await analyzeOnly(entryPoint, memorySourceFiles, printSteps: true);
@@ -162,8 +162,8 @@ Future<ResultKind> mainInternal(List<String> args,
   checkElementEnvironment(environment1, environment2, equivalence);
 
   ResolutionEnqueuer enqueuer2 = compiler2.enqueuer.resolution;
-  BackendUsage backendUsage2 = compiler2.backend.backendUsage;
   ClosedWorld closedWorld2 = compiler2.resolutionWorldBuilder.closeWorld();
+  BackendUsage backendUsage2 = closedWorld2.backendUsage;
 
   checkNativeClasses(compiler1, compiler2, equivalence);