Move closed world reasoning methods from ClassWorld to ClosedWorld.

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

Index: pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
diff --git a/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart b/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
index 86a24996d261b12091a0504d585408f3a9ab6317..b5e3c713195b95b7326645d6f309eb00b1bd281c 100644
--- a/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
+++ b/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
@@ -118,7 +118,7 @@ class _FieldNamingScope {
   }
 
   factory _FieldNamingScope.forClass(
-      ClassElement cls, ClassWorld world, _FieldNamingRegistry registry) {
+      ClassElement cls, ClosedWorld world, _FieldNamingRegistry registry) {
     _FieldNamingScope result = registry.scopes[cls];
     if (result != null) return result;