Hide Symbol implementation in a closure.

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";
 
 // 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;
 
 DefaultObjectToString = ObjectToStringHarmony;
 // ES6 draft 08-24-14, section 19.1.3.6
 function ObjectToStringHarmony() {
   if (IS_UNDEFINED(this) && !IS_UNDETECTABLE(this)) return "[object Undefined]";
   if (IS_NULL(this)) return "[object Null]";
   var O = ToObject(this);
   var builtinTag = %_ClassOf(O);
   var tag = O[symbolToStringTag];
   if (!IS_STRING(tag)) {
     tag = builtinTag;
   }
   return "[object " + tag + "]";
 }
 
 function HarmonyToStringExtendSymbolPrototype() {
   %CheckIsBootstrapping();
 
-  InstallConstants($Symbol, $Array(
+  InstallConstants(global.Symbol, $Array(
     // TODO(dslomov, caitp): Move to symbol.js when shipping
    "toStringTag", symbolToStringTag
   ));
 }
 
 HarmonyToStringExtendSymbolPrototype();
 
 function HarmonyToStringExtendObjectPrototype() {
   %CheckIsBootstrapping();
 
   // Can't use InstallFunctions() because will fail in Debug mode.
   // Emulate InstallFunctions() here.
   %FunctionSetName(ObjectToStringHarmony, "toString");
   %FunctionRemovePrototype(ObjectToStringHarmony);
   %SetNativeFlag(ObjectToStringHarmony);
 
   // 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();