Consistently use "use strict" where possible.

src/string-iterator.js

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-'use strict';
+"use strict";
 
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $String = global.String;
 
 
 var stringIteratorIteratedStringSymbol =
     GLOBAL_PRIVATE("StringIterator#iteratedString");
 var stringIteratorNextIndexSymbol = GLOBAL_PRIVATE("StringIterator#next");
@@ skipping 84 matching lines @@
 
 
 function ExtendStringPrototypeWithIterator() {
   %CheckIsBootstrapping();
 
   %FunctionSetName(StringPrototypeIterator, '[Symbol.iterator]');
   %AddNamedProperty($String.prototype, symbolIterator,
                     StringPrototypeIterator, DONT_ENUM);
 }
 ExtendStringPrototypeWithIterator();