Fix issue https://code.google.com/p/dart/issues/detail?id=9251, by compiling for/in correctly.

tests/language/for_in2_test.dart

+// Copyright (c) 2013, 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.
+
+// Test that using a field or a global variable for a for/in variable
+// works.
+
+import "package:expect/expect.dart";
+
+Set<int> set = new Set.from([1, 2]);
+var x;
+
+class A {
+  var field;
+  test() {
+    int count = 0;
+    for (field in set) {
+      count += field;
+    }
+    Expect.equals(3, count);
+
+    count = 0;
+    for (x in set) {
+      count += x;
+    }
+    Expect.equals(3, count);
+  }
+}
+
+void main() {
+  new A().test();
+}