Split World usage into open, inference, and closed world.

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 7ac77d5e0c7f2a2fd11b16f751457f497e27cad1..86a24996d261b12091a0504d585408f3a9ab6317 100644
--- a/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
+++ b/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
@@ -24,8 +24,8 @@ abstract class _MinifiedFieldNamer implements Namer {
       names = new _FieldNamingScope.forBox(element.box, fieldRegistry);
     } else {
       ClassElement cls = element.enclosingClass;
-      names =
-          new _FieldNamingScope.forClass(cls, compiler.world, fieldRegistry);
+      names = new _FieldNamingScope.forClass(
+          cls, compiler.closedWorld, fieldRegistry);
     }
 
     if (names.containsField(element)) {