Add boxing for modified variables

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

 // Copyright (c) 2016, 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 '../closure.dart';
 import '../common.dart';
 import '../elements/elements.dart';
 import '../elements/entities.dart';
 import '../elements/types.dart';
 import '../io/source_information.dart';
@@ skipping 137 matching lines @@
       // The box is passed as a parameter to a generative
       // constructor body.
       box = builder.addParameter(closureInfo.context, commonMasks.nonNullType);
     } else {
       box = createBox();
     }
     // Add the box to the known locals.
     directLocals[closureInfo.context] = box;
     // Make sure that accesses to the boxed locals go into the box. We also
     // need to make sure that parameters are copied into the box if necessary.
-    closureInfo.forEachBoxedVariable((_from, _to) {
-      LocalVariableElement from = _from;
-      BoxFieldElement to = _to;
+    closureInfo.forEachBoxedVariable((Local from, FieldEntity to) {
       // The [from] can only be a parameter for function-scopes and not
       // loop scopes.
       if (from.isRegularParameter && !forGenerativeConstructorBody) {
         // Now that the redirection is set up, the update to the local will
         // write the parameter value into the box.
         // Store the captured parameter in the box. Get the current value
         // before we put the redirection in place.
         // We don't need to update the local for a generative
         // constructor body, because it receives a box that already
         // contains the updates as the last parameter.
@@ skipping 522 matching lines @@
   final MemberEntity memberContext;
 
   // Avoid slow Object.hashCode.
   final int hashCode = _nextHashCode = (_nextHashCode + 1).toUnsigned(30);
   static int _nextHashCode = 0;
 
   SyntheticLocal(this.name, this.executableContext, this.memberContext);
 
   toString() => 'SyntheticLocal($name)';
 }