Extract BackendUsageBuilder from BackendUsage

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;
 import '../elements/elements.dart';
 import '../elements/entities.dart';
 import '../elements/modelx.dart' show FunctionElementX;
 import '../elements/resolution_types.dart';
 import '../js_backend/backend_helpers.dart' show BackendHelpers;
+import '../js_backend/backend_usage.dart' show BackendUsageBuilder;
 import '../js_backend/js_backend.dart';
 import '../js_emitter/js_emitter.dart' show CodeEmitterTask, NativeEmitter;
 import 'package:front_end/src/fasta/scanner.dart' show BeginGroupToken, Token;
 import 'package:front_end/src/fasta/scanner.dart' as Tokens show EOF_TOKEN;
 import '../tree/tree.dart';
 import '../universe/use.dart' show StaticUse, TypeUse;
 import '../universe/world_impact.dart'
     show WorldImpact, WorldImpactBuilder, WorldImpactBuilderImpl;
 import 'behavior.dart';
 
@@ skipping 48 matching lines @@
 
   final Compiler compiler;
   final bool enableLiveTypeAnalysis;
 
   /// Subclasses of [NativeEnqueuerBase] are constructed by the backend.
   NativeEnqueuerBase(Compiler compiler, this.enableLiveTypeAnalysis)
       : this.compiler = compiler;
 
   JavaScriptBackend get backend => compiler.backend;
   BackendHelpers get helpers => backend.helpers;
+  BackendUsageBuilder get _backendUsageBuilder => backend.backendUsageBuilder;
   Resolution get resolution => compiler.resolution;
 
   DiagnosticReporter get reporter => compiler.reporter;
   CommonElements get commonElements => compiler.commonElements;
 
   void onInstantiatedType(ResolutionInterfaceType type) {
     if (_unusedClasses.remove(type.element)) {
       _registeredClasses.add(type.element);
     }
   }
@@ skipping 343 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.backendUsage.registerBackendUse(element);
-      backend.backendUsage.registerGlobalDependency(element);
+      _backendUsageBuilder.registerBackendUse(element);
+      _backendUsageBuilder.registerGlobalDependency(element);
     }
 
     staticUse(helpers.defineProperty);
     staticUse(helpers.toStringForNativeObject);
     staticUse(helpers.hashCodeForNativeObject);
     staticUse(helpers.closureConverter);
     return _findNativeExceptions();
   }
 
   Iterable<ClassElement> _findNativeExceptions() {
@@ skipping 146 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.');
   }
 }