Add @@iterator support for strings

test/mjsunit/harmony/iteration-semantics.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 201 matching lines @@
                            one_time_getter({ done: false }, 'value', 2),
                            { value: 37, done: true },
                            never_getter(never_getter({}, 'done'), 'value')])));
 
 // Null and undefined do not cause an error.
 assertEquals(0, fold(sum, 0, unreachable(null)));
 assertEquals(0, fold(sum, 0, unreachable(undefined)));
 
 // Other non-iterators do cause an error.
 assertThrows('fold(sum, 0, unreachable({}))', TypeError);
-assertThrows('fold(sum, 0, unreachable("foo"))', TypeError);
+assertThrows('fold(sum, 0, unreachable(false))', TypeError);
 assertThrows('fold(sum, 0, unreachable(37))', TypeError);
 
 // "next" is looked up each time.
 assertThrows('fold(sum, 0, remove_next_after(integers_until(10), 5))',
              TypeError);
 // It is not called at any other time.
 assertEquals(45,
              fold(sum, 0, remove_next_after(integers_until(10), 10)));
 // It is not looked up too many times.
 assertEquals(45,
@@ skipping 96 matching lines @@
       if (name == 'next' && n-- < 0) throw "unreachable";
       return iterator[name];
     },
     // Needed for integers_until(10)'s this.n++.
     set: function(receiver, name, val) {
       return iterator[name] = val;
     }
   }));
 }
 assertEquals(45, fold(sum, 0, poison_proxy_after(integers_until(10), 10)));