Improved JSON stringify.

src/array.js

Index: src/array.js
===================================================================
--- src/array.js	(revision 5918)
+++ src/array.js	(working copy)
@@ -148,6 +148,9 @@
         }
       }
     }
+    elements.length = elements_length;
+    var result = %_FastAsciiArrayJoin(elements, "");
+    if (!IS_UNDEFINED(result)) return result;
     return %StringBuilderConcat(elements, elements_length, '');
   } finally {
     // Make sure to pop the visited array no matter what happens.
@@ -366,7 +369,7 @@
   }
 
   var result = %_FastAsciiArrayJoin(this, separator);
-  if (typeof result != "undefined") return result;
+  if (!IS_UNDEFINED(result)) return result;
 
   var length = TO_UINT32(this.length);
   return Join(this, length, separator, ConvertToString);