Add boxing for modified variables

tests/compiler/dart2js/closure/closure_test.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:io' hide Link;
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/closure.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/common.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/diagnostics/diagnostic_listener.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/elements/entities.dart';
 import 'package:compiler/src/kernel/element_map.dart';
 import 'package:compiler/src/kernel/kernel_backend_strategy.dart';
 import 'package:compiler/src/js_model/locals.dart';
 import 'package:compiler/src/tree/nodes.dart' as ast;
 import 'package:compiler/src/util/util.dart';
 import 'package:expect/expect.dart';
 import '../equivalence/id_equivalence.dart';
 import '../equivalence/id_equivalence_helper.dart';
 import 'package:kernel/ast.dart' as ir;
 
 main(List<String> args) {
   bool verbose = args.contains('-v');
   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);
   });
 }
 
 /// Compute closure data mapping for [_member] as a [MemberElement].
 ///
 /// Fills [actualMap] with the data and [sourceSpanMap] with the source spans
 /// for the data origin.
 void computeClosureData(
@@ skipping 274 matching lines @@
         if (value != '') {
           sb.write('=');
           sb.write(value);
         }
         needsComma = true;
       }
     }
     return sb.toString();
   }
 }