Add ElementCodegenWorldBuilder

pkg/compiler/lib/src/js_backend/backend.dart

 // Copyright (c) 2012, 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.
 
 library js_backend.backend;
 
 import '../common.dart';
 import '../common/backend_api.dart'
     show ForeignResolver, NativeRegistry, ImpactTransformer;
 import '../common/codegen.dart' show CodegenImpact, CodegenWorkItem;
@@ skipping 875 matching lines @@
         compiler.elementEnvironment,
         commonElements,
         compiler.frontEndStrategy.dartTypes,
         emitter,
         _nativeResolutionEnqueuer,
         closedWorld.nativeData);
     return new CodegenEnqueuer(
         task,
         compiler.options,
         const TreeShakingEnqueuerStrategy(),
-        new CodegenWorldBuilderImpl(
-            nativeBasicData, closedWorld, constants, const TypeMaskStrategy()),
-        new CodegenWorkItemBuilder(this, closedWorld, compiler.options),
+        compiler.backendStrategy.createCodegenWorldBuilder(
+            nativeBasicData, closedWorld, const TypeMaskStrategy()),
+        compiler.backendStrategy.createCodegenWorkItemBuilder(closedWorld),
         new CodegenEnqueuerListener(
             compiler.elementEnvironment,
             commonElements,
             impacts,
             closedWorld.backendUsage,
             rtiNeed,
             customElementsCodegenAnalysis,
             typeVariableCodegenAnalysis,
             lookupMapAnalysis,
             mirrorsCodegenAnalysis,
@@ skipping 523 matching lines @@
 
   bool canUseAliasedSuperMember(MemberEntity member, Selector selector) {
     return !selector.isGetter;
   }
 
   /// Returns `true` if [member] is called from a subclass via `super`.
   bool isAliasedSuperMember(MemberEntity member) {
     return _aliasedSuperMembers.contains(member);
   }
 }