Avoid boxing loop variables that are only mutated in initializer or update

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'''
+main() {
+  var a;
+  for (var i=0; i<10; i++) {
+    a = () => i;
+  }
+  print(a());
+}
+''';
+
+String NEGATIVE_TEST = r'''
+run(f) => f();
+main() {
+  var a;
+  for (var i=0; i<10; run(() => i++)) {
+    a = () => i;
+  }
+  print(a());
+}
+''';
+
+main() {
+  asyncTest(() => compileAll(TEST).then((generated) {
+    Expect.isTrue(generated.contains('main_closure(i)'),
+        'for-loop variable was boxed');
+  }));
+  asyncTest(() => compileAll(NEGATIVE_TEST).then((generated) {
+    Expect.isFalse(generated.contains('main_closure(i)'),
+        'for-loop variable was not boxed');
+  }));
+}