Use the PushIfAbsent function for the JSON stringify stack....

src/runtime.cc

Index: src/runtime.cc
===================================================================
--- src/runtime.cc	(revision 5940)
+++ src/runtime.cc	(working copy)
@@ -7671,13 +7671,13 @@
 }
 
 
-// Push an array unto an array of arrays if it is not already in the
+// Push an object unto an array of objects if it is not already in the
 // array.  Returns true if the element was pushed on the stack and
 // false otherwise.
 static MaybeObject* Runtime_PushIfAbsent(Arguments args) {
   ASSERT(args.length() == 2);
   CONVERT_CHECKED(JSArray, array, args[0]);
-  CONVERT_CHECKED(JSArray, element, args[1]);
+  CONVERT_CHECKED(JSObject, element, args[1]);
   RUNTIME_ASSERT(array->HasFastElements());
   int length = Smi::cast(array->length())->value();
   FixedArray* elements = FixedArray::cast(array->elements());