Add boxing for modified 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 eae9c91c04adbc772c6e261de0e28e0c344f545f..0660cd6f389de6e11be3eef4e64cdbc0490cefb7 100644
--- a/tests/compiler/dart2js/closure/closure_test.dart
+++ b/tests/compiler/dart2js/closure/closure_test.dart
@@ -26,13 +26,7 @@ main(List<String> args) {
   asyncTest(() async {
     Directory dataDir = new Directory.fromUri(Platform.script.resolve('data'));
     await checkTests(dataDir, computeClosureData, computeKernelClosureData,
-        // TODO(johnnniwinther,efortuna): Enable these tests for .dill.
-        skipForKernel: [
-          'captured_variable.dart',
-          'mixed.dart',
-          'mutations.dart',
-          'nested_closures.dart'
-        ],
+        skipForKernel: [],
         options: [Flags.disableTypeInference],
         verbose: verbose);
   });