Extract ClosedWorldRefiner implementation from ClosedWorldImpl to ClosedWorldBase

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

Index: pkg/compiler/lib/src/js_backend/enqueuer.dart
diff --git a/pkg/compiler/lib/src/js_backend/enqueuer.dart b/pkg/compiler/lib/src/js_backend/enqueuer.dart
index b1c76c856192391b94559849c957832a7a58f3cf..255bc9e44fa4e8b8ef65f54ebeea54a96ca63431 100644
--- a/pkg/compiler/lib/src/js_backend/enqueuer.dart
+++ b/pkg/compiler/lib/src/js_backend/enqueuer.dart
@@ -51,7 +51,7 @@ class CodegenEnqueuer extends EnqueuerImpl {
   final Queue<WorkItem> _queue = new Queue<WorkItem>();
 
   /// All declaration elements that have been processed by codegen.
-  final Set<Entity> _processedEntities = new Set<Entity>();
+  final Set<MemberEntity> _processedEntities = new Set<MemberEntity>();
 
   static const ImpactUseCase IMPACT_USE =
       const ImpactUseCase('CodegenEnqueuer');
@@ -259,7 +259,7 @@ class CodegenEnqueuer extends EnqueuerImpl {
   ImpactUseCase get impactUse => IMPACT_USE;
 
   @override
-  Iterable<Entity> get processedEntities => _processedEntities;
+  Iterable<MemberEntity> get processedEntities => _processedEntities;
 
   @override
   Iterable<ClassEntity> get processedClasses => _worldBuilder.processedClasses;