Fix 15.2.3.4 es5conform test

src/v8natives.js

 // Copyright 2006-2008 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 664 matching lines @@
     if (namedInterceptorNames) {
       propertyNames = propertyNames.concat(namedInterceptorNames);
     }
   }
 
   // Property names are expected to be unique strings.
   var propertySet = {};
   var j = 0;
   for (var i = 0; i < propertyNames.length; ++i) {
     var name = ToString(propertyNames[i]);
-    if (name in propertySet)
+    // We need to check for the exact property value since for intrinsic
+    // properties like toString if(propertySet["toString"]) will always
+    // succeed.
+    if (propertySet[name] === true)
       continue;
     propertySet[name] = true;
     propertyNames[j++] = name;
   }
   propertyNames.length = j;
 
   return propertyNames;
 }
 
 
@@ skipping 342 matching lines @@
 
 // ----------------------------------------------------------------------------
 
 function SetupFunction() {
   InstallFunctions($Function.prototype, DONT_ENUM, $Array(
     "toString", FunctionToString
   ));
 }
 
 SetupFunction();