Access BackendUsage through ClosedWorld

tests/compiler/dart2js/kernel/closed_world2_test.dart

 // Copyright (c) 2017, 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.
 
 // Partial test that the closed world computed from [WorldImpact]s derived from
 // kernel is equivalent to the original computed from resolution.
 library dart2js.kernel.closed_world2_test;
 
 import 'dart:async';
 
@@ skipping 124 matching lines @@
   if (collector.errors.isNotEmpty && skipErrors) {
     print('Skipping due to errors.');
     return ResultKind.errors;
   }
   if (collector.warnings.isNotEmpty && skipWarnings) {
     print('Skipping due to warnings.');
     return ResultKind.warnings;
   }
   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);
   Compiler compiler = compilers.a;
   compiler.resolutionWorldBuilder.closeWorld();
   ElementEnvironment environment1 = compiler.elementEnvironment;
 
   Compiler compiler2 = compilers.b;
   KernelFrontEndStrategy frontEndStrategy = compiler2.frontEndStrategy;
   KernelToElementMapImpl elementMap = frontEndStrategy.elementMap;
   Expect.isFalse(compiler2.compilationFailed);
 
   KernelEquivalence equivalence = new KernelEquivalence(elementMap);
 
   ElementEnvironment environment2 = compiler2.elementEnvironment;
   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);
 
   checkBackendUsage(backendUsage1, backendUsage2, equivalence);
 
   checkResolutionEnqueuers(backendUsage1, backendUsage2, enqueuer1, enqueuer2,
       elementEquivalence: equivalence.entityEquivalence,
       typeEquivalence: (ResolutionDartType a, DartType b) {
     return equivalence.typeEquivalence(unalias(a), b);
   }, elementFilter: elementFilter, verbose: arguments.verbose);
@@ skipping 185 matching lines @@
       }, equivalence.entityEquivalence);
 
       classes2.add(cls2);
     });
     env2.forEachClass(lib2, (ClassEntity cls2) {
       Expect.isTrue(classes2.contains(cls2), "Extra class $cls2 in $lib2");
     });
   });
   // TODO(johnniwinther): Test the remaining properties of [ElementEnvironment].
 }