Add boxing for modified variables

pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart

Index: pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
diff --git a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
index eae2f4b1e208779a7782f0fd624cf55ece493341..243e5c48a9a1a78055eb13df0687411068004e52 100644
--- a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
+++ b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
@@ -18,6 +18,7 @@ import '../elements/resolution_types.dart';
 import '../elements/types.dart';
 import '../js_backend/js_backend.dart';
 import '../js_backend/native_data.dart';
+import '../js_model/closure.dart' show KernelScopeInfo, JRecordField;
 import '../kernel/element_map.dart';
 import '../kernel/element_map_mixins.dart';
 import '../kernel/kernel.dart';
@@ -410,6 +411,12 @@ class KernelAstAdapter extends KernelToElementMapBaseMixin
     LocalFunctionElement localFunction = getElement(node);
     return closureLookup.getClosureInfo(localFunction.node);
   }
+
+  @override
+  Map<Local, JRecordField> makeRecordContainer(
+      KernelScopeInfo info, MemberEntity member, KernelToLocalsMap localsMap) {
+    throw new UnsupportedError('KernelAstAdapter.makeRecordContainer');
+  }
 }
 
 /// Visitor that converts kernel dart types into [ResolutionDartType].