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

src/runtime.cc

 // Copyright 2010 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 7653 matching lines @@
   Object* allocation;
   { MaybeObject* maybe_allocation = Heap::new_space()->AllocateRaw(size);
     if (maybe_allocation->ToObject(&allocation)) {
       Heap::CreateFillerObjectAt(HeapObject::cast(allocation)->address(), size);
     }
     return maybe_allocation;
   }
 }
 
 
-// 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());
   for (int i = 0; i < length; i++) {
     if (elements->get(i) == element) return Heap::false_value();
   }
   Object* obj;
   { MaybeObject* maybe_obj = array->SetFastElement(length, element);
     if (!maybe_obj->ToObject(&obj)) return maybe_obj;
   }
@@ skipping 3093 matching lines @@
   } else {
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     Counters::gc_last_resort_from_js.Increment();
     Heap::CollectAllGarbage(false);
   }
 }
 
 
 } }  // namespace v8::internal