Consistently use "use strict" where possible.

src/harmony-tostring.js

 // Copyright 2013 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 and symbol.js:
 // var $Object = global.Object;
 // var $Symbol = global.Symbol;
 
 var kBuiltinStringTags = {
   "__proto__": null,
   "Arguments": true,
   "Array": true,
@@ skipping 47 matching lines @@
   // Set up the non-enumerable functions on the Array prototype object.
   var desc = ToPropertyDescriptor({
     value: ObjectToStringHarmony
   });
   DefineOwnProperty($Object.prototype, "toString", desc, false);
 
   %ToFastProperties($Object.prototype);
 }
 
 HarmonyToStringExtendObjectPrototype();