Add more tests for boxed variables.

tests/compiler/dart2js/closure/closure_test.dart

Index: tests/compiler/dart2js/closure/closure_test.dart
diff --git a/tests/compiler/dart2js/closure/closure_test.dart b/tests/compiler/dart2js/closure/closure_test.dart
index a206a1102197b4c8246585e8a6160c7db965b492..79a6a2d0d7a9d6171a948d3941c9f21644054bc8 100644
--- a/tests/compiler/dart2js/closure/closure_test.dart
+++ b/tests/compiler/dart2js/closure/closure_test.dart
@@ -20,6 +20,12 @@ import '../equivalence/id_equivalence.dart';
 import '../equivalence/id_equivalence_helper.dart';
 import 'package:kernel/ast.dart' as ir;
 
+// TODO(johnnniwinther,efortuna): Enable the these tests for .dill.
+const List<String> whiteList = const <String>[
+  'nested_closures.dart',
+  'mixed.dart',
+];
+
 main(List<String> args) {
   bool verbose = args.contains('-v');
   asyncTest(() async {
@@ -32,8 +38,7 @@ main(List<String> args) {
       print('--from source---------------------------------------------------');
       await checkCode(annotatedCode, computeClosureData, compileFromSource,
           verbose: verbose);
-      // TODO(johnnniwinther,efortuna): Enable the these tests for .dill.
-      if (['nested_closures.dart'].contains(entity.uri.pathSegments.last)) {
+      if (whiteList.contains(entity.uri.pathSegments.last)) {
         print('--skipped for dill--------------------------------------------');
         continue;
       }