Added for-loop variable tracking and regular closures/initializers captured variable tracking.

tests/compiler/dart2js/forloop_box_test.dart

 // Copyright (c) 2015, 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 "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 import 'compiler_helper.dart';
 
-String TEST = r'''
+String SHOULD_NOT_BE_BOXED_TEST = r'''
 main() {
   var a;
   for (var i=0; i<10; i++) {
     a = () => i;
   }
   print(a());
 }
 ''';
 
-String NEGATIVE_TEST = r'''
+String SHOULD_BE_BOXED_TEST = r'''
 run(f) => f();
 main() {
   var a;
   for (var i=0; i<10; run(() => i++)) {
     a = () => i;
   }
   print(a());
 }
 ''';
 
+String ONLY_UPDATE_LOOP_VAR_TEST = r'''
+run(f) => f();
+main() {
+  var a;
+  for (var i=0; i<10; run(() => i++)) {
+    var b = 3;
+    a = () => b = i;
+  }
+  print(a());
+}
+''';
+
 main() {
-  asyncTest(() => compileAll(TEST).then((generated) {
+  asyncTest(() => compileAll(SHOULD_NOT_BE_BOXED_TEST).then((generated) {
         Expect.isTrue(generated.contains('main_closure(i)'),
             'for-loop variable was boxed');

[Siggi Cherem (dart-lang), 2017/06/30 22:02:10]

while we are here, let's update the failure reason also to say what it should
do, instead of what happened.

(e.g. here "... variable should not be boxed")

[Emily Fortuna, 2017/06/30 23:48:10]

Done.

       }));
-  asyncTest(() => compileAll(NEGATIVE_TEST).then((generated) {
+  asyncTest(() => compileAll(SHOULD_BE_BOXED_TEST).then((generated) {
         Expect.isFalse(generated.contains('main_closure(i)'),
             'for-loop variable was not boxed');
       }));
+  asyncTest(() => compileAll(ONLY_UPDATE_LOOP_VAR_TEST).then((generated) {
+        Expect.isFalse(generated.contains('main_closure(i)'),
+            'for-loop variable was not boxed');
+        Expect.isFalse(generated.contains(', _box_0.b = 3,'),
+            'non for-loop captured variable should not be updated in loop');
+      }));
 }