Add boxing for modified variables

pkg/compiler/lib/src/js_model/js_strategy.dart

Index: pkg/compiler/lib/src/js_model/js_strategy.dart
diff --git a/pkg/compiler/lib/src/js_model/js_strategy.dart b/pkg/compiler/lib/src/js_model/js_strategy.dart
index 5ce05da155e45e996f4cfaca661a16b6ae101c2e..a7c2a25cfc3b5c9757344c331326a22437877c2d 100644
--- a/pkg/compiler/lib/src/js_model/js_strategy.dart
+++ b/pkg/compiler/lib/src/js_model/js_strategy.dart
@@ -419,6 +419,7 @@ class JsClosedWorld extends ClosedWorldBase with KernelClosedWorldMixin {
       MemberEntity member,
       ir.FunctionNode originalClosureFunctionNode,
       JLibrary enclosingLibrary,
+      Map<Local, JRecordField> boxedVariables,
       KernelScopeInfo info,
       ir.Location location,
       KernelToLocalsMap localsMap) {
@@ -428,6 +429,7 @@ class JsClosedWorld extends ClosedWorldBase with KernelClosedWorldMixin {
         member,
         originalClosureFunctionNode,
         enclosingLibrary,
+        boxedVariables,
         info,
         location,
         localsMap,