Add boxing for modified variables

pkg/compiler/lib/src/kernel/element_map.dart

Index: pkg/compiler/lib/src/kernel/element_map.dart
diff --git a/pkg/compiler/lib/src/kernel/element_map.dart b/pkg/compiler/lib/src/kernel/element_map.dart
index e8c9139cd2fc823bd01bc7e3f889347cd047c1ef..97c73a0457252d52675720c9b6cca7ac73d99404 100644
--- a/pkg/compiler/lib/src/kernel/element_map.dart
+++ b/pkg/compiler/lib/src/kernel/element_map.dart
@@ -16,6 +16,7 @@ import '../js/js.dart' as js;
 import '../js_backend/namer.dart';
 import '../js_backend/native_data.dart';
 import '../js_emitter/code_emitter_task.dart';
+import '../js_model/closure.dart' show JRecordField, KernelScopeInfo;
 import '../native/native.dart' as native;
 import '../types/types.dart';
 import '../universe/call_structure.dart';
@@ -203,6 +204,11 @@ abstract class KernelToElementMapForBuilding implements KernelToElementMap {
   // TODO(johnniwinther): Avoid this method by deriving the uri directly from
   // the node.
   String getDeferredUri(ir.LibraryDependency node);
+
+  /// Make a record to ensure variables that are are declared in one scope and
+  /// modified in another get their values updated correctly.
+  Map<Local, JRecordField> makeRecordContainer(
+      KernelScopeInfo info, MemberEntity member, KernelToLocalsMap localsMap);
 }
 
 // TODO(johnniwinther,efortuna): Add more when needed.