Add boxing for modified variables

pkg/compiler/lib/src/js_model/js_strategy.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.
 
 library dart2js.js_model.strategy;
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../closure.dart' show ClosureConversionTask;
 import '../common.dart';
@@ skipping 401 matching lines @@
             typesImplementedBySubclasses,
             classHierarchyNodes,
             classSets);
 
   /// Construct a closure class and set up the necessary class inference
   /// hierarchy.
   KernelClosureClass buildClosureClass(
       MemberEntity member,
       ir.FunctionNode originalClosureFunctionNode,
       JLibrary enclosingLibrary,
+      Map<Local, JRecordField> boxedVariables,
       KernelScopeInfo info,
       ir.Location location,
       KernelToLocalsMap localsMap) {
     ClassEntity superclass = commonElements.closureClass;
 
     KernelClosureClass cls = elementMap.constructClosureClass(
         member,
         originalClosureFunctionNode,
         enclosingLibrary,
+        boxedVariables,
         info,
         location,
         localsMap,
         new InterfaceType(superclass, const []));
 
     // Tell the hierarchy that this is the super class. then we can use
     // .getSupertypes(class)
     ClassHierarchyNode parentNode = getClassHierarchyNode(superclass);
     ClassHierarchyNode node = new ClassHierarchyNode(
         parentNode, cls.closureClassEntity, getHierarchyDepth(superclass) + 1);
     addClassHierarchyNode(cls.closureClassEntity, node);
     for (InterfaceType type in getOrderedTypeSet(superclass).types) {
       // TODO(efortuna): assert that the FunctionClass is in this ordered set.
       // If not, we need to explicitly add node as a subtype of FunctionClass.
       ClassSet subtypeSet = getClassSet(type.element);
       subtypeSet.addSubtype(node);
     }
     addClassSet(cls.closureClassEntity, new ClassSet(node));
     node.isDirectlyInstantiated = true;
 
     return cls;
   }
 
   @override
   void registerClosureClass(ClassEntity cls) {
     throw new UnsupportedError('JsClosedWorld.registerClosureClass');
   }
 }