Handle const loop variable.

tests/language/const_for_in_variable_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";
-
-class A {
-  call({a: 42}) {
-    return 499 + a;
+main() {
+  for(
+      const                           /// 01: compile-time error
+      final                           /// 02: ok
+      int x in const [1, 2, 3]) {
+    break;
   }
-}
-
-main() {
-  Expect.equals(497, Function.apply(new A(), [], {#a: -2}));
-}
+}