Add EnumSet.

pkg/compiler/lib/src/resolution/registry.dart

Index: pkg/compiler/lib/src/resolution/registry.dart
diff --git a/pkg/compiler/lib/src/resolution/registry.dart b/pkg/compiler/lib/src/resolution/registry.dart
index 5fc06e1f13f5d345974e0505fff4d94207f46d8f..b3c7686a01e8af07a73eea221be031de2ebdf0f5 100644
--- a/pkg/compiler/lib/src/resolution/registry.dart
+++ b/pkg/compiler/lib/src/resolution/registry.dart
@@ -36,7 +36,10 @@ import '../universe/use.dart' show
     TypeUse;
 import '../universe/world_impact.dart' show
     WorldImpactBuilder;
-import '../world.dart' show World;
+import '../util/enumset.dart' show
+    EnumSet;
+import '../world.dart' show
+    World;
 
 import 'send_structure.dart';
 
@@ -47,7 +50,7 @@ import 'tree_elements.dart' show
 
 class _ResolutionWorldImpact extends ResolutionImpact with WorldImpactBuilder {
   final String name;
-  Setlet<Feature> _features;
+  EnumSet<Feature> _features;
   Setlet<MapLiteralUse> _mapLiterals;
   Setlet<ListLiteralUse> _listLiterals;
   Setlet<String> _constSymbolNames;
@@ -98,14 +101,15 @@ class _ResolutionWorldImpact extends ResolutionImpact with WorldImpactBuilder {
 
   void registerFeature(Feature feature) {
     if (_features == null) {
-      _features = new Setlet<Feature>();
+      _features = new EnumSet<Feature>();
     }
     _features.add(feature);
   }
 
   @override
   Iterable<Feature> get features {
-    return _features != null ? _features : const <Feature>[];
+    return _features != null
+        ? _features.iterable(Feature.values) : const <Feature>[];
   }
 
   void registerConstantLiteral(ConstantExpression constant) {