Fix the locals lookup of variables and partial implementation of boxing of variables.

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 3405339113f3890066fbbaf48308796da24d1a94..08ef74a4c84935cd9a3c063e1bcd24242b519d35 100644
--- a/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
+++ b/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
@@ -21,7 +21,7 @@ abstract class _MinifiedFieldNamer implements Namer {
     _FieldNamingScope names;
     if (element is BoxFieldElement) {
       names = new _FieldNamingScope.forBox(element.box, fieldRegistry);
-    } else if (element is JBoxedField) {
+    } else if (element is JRecord) {
       names = new _FieldNamingScope.forBox(element.box, fieldRegistry);
     } else {
       ClassEntity cls = element.enclosingClass;