Don't allow to iterate over strings.

tests/language/for_in3_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:expect/expect.dart";
 
-class C<D> {
-  void set D(int value) {
-    field = value;
-  }
-
-  int field;
-}
+// Dart test for testing that strings aren't iterable.
 
 main() {
-  C<int> c = new C<int>();
-  c.D = 1;
-  Expect.equals(c.field, 1);
+  Expect.throws(() {
+    var chars = [];
+    for (var c in "foo") chars.add(c);
+  }, (e) => e is NoSuchMethodError);
 }