Use for-in loop variable name if possible

tests/compiler/dart2js/kernel/loops_test.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 'package:test/test.dart';
 
 import 'helper.dart' show check;
 
 main() {
   test('for loop', () {
@@ skipping 34 matching lines @@
 
   test('for-in loop', () {
     String code = '''
 main() {
   var sum = 0;
   for (var a in [1, 2, 3]) {
     sum += a;
   }
   return sum;
 }''';
-    // TODO(het): Check that the code is alpha-equivalent. That is,
-    // the same except for variable names.
     return check(code);
-  }, skip: "The output is the same, with one variable renamed");
+  });
 
   test('for-in loop optimized', () {
     String code = '''
 main() {
   var sum = 0;
   for (var a in [1, 2, 3]) {
     sum += a;
   }
   return sum;
 }''';
@@ skipping 26 matching lines @@
       sum += a;
       if (a + i < 5)
         break outer;
     }
   }
   return sum;
 }''';
     return check(code, disableTypeInference: false);
   });
 }