Create Namer and Emitter on codegen start.

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 b5e3c713195b95b7326645d6f309eb00b1bd281c..d7a3ca966f64c5227914f837e6c97aa87be022c5 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 +24,7 @@ abstract class _MinifiedFieldNamer implements Namer {
       names = new _FieldNamingScope.forBox(element.box, fieldRegistry);
     } else {
       ClassElement cls = element.enclosingClass;
-      names = new _FieldNamingScope.forClass(
-          cls, compiler.closedWorld, fieldRegistry);
+      names = new _FieldNamingScope.forClass(cls, closedWorld, fieldRegistry);
     }
 
     if (names.containsField(element)) {