Refactor BackendHelpers to be reusageable with KernelWorldBuilder

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/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 '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';
 
 /**
  * This could be an abstract class but we use it as a stub for the dart_backend.
  */
 class NativeEnqueuer {
@@ skipping 607 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.');
   }
 }