Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/native/enqueue.dart

 // Copyright (c) 2014, 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.
 
 import '../common.dart';
 import '../common/backend_api.dart' show ForeignResolver;
 import '../common/resolution.dart' show Resolution;
 import '../compiler.dart' show Compiler;
 import '../constants/values.dart';
 import '../core_types.dart' show CommonElements;
@@ skipping 426 matching lines @@
   }
 
   Iterable<ClassElement> _findUnusedClassesMatching(
       bool predicate(classElement)) {
     return _unusedClasses.where(predicate);
   }
 
   Iterable<ClassElement> _onFirstNativeClass(WorldImpactBuilder impactBuilder) {
     void staticUse(element) {
       impactBuilder.registerStaticUse(new StaticUse.foreignUse(element));
-      backend.registerBackendUse(element);
+      backend.backendUsage.registerBackendUse(element);
       compiler.globalDependencies.registerDependency(element);
     }
 
     staticUse(helpers.defineProperty);
     staticUse(helpers.toStringForNativeObject);
     staticUse(helpers.hashCodeForNativeObject);
     staticUse(helpers.closureConverter);
     return _findNativeExceptions();
   }
 
@@ skipping 147 matching lines @@
     List<ClassEntity> directSubtypes =
         emitter.directSubtypes.putIfAbsent(superclass, () => <ClassEntity>[]);
     directSubtypes.add(cls);
   }
 
   void logSummary(log(message)) {
     log('Compiled ${_registeredClasses.length} native classes, '
         '${_unusedClasses.length} native classes omitted.');
   }
 }