Don't fast RemoveArrayHoles in case of arguments arrays.

src/array.js

 // Copyright 2012 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 1097 matching lines @@
     // the prototype chain on non-Array objects.
     // We do this by copying them to this object and sorting only
     // local elements. This is not very efficient, but sorting with
     // inherited elements happens very, very rarely, if at all.
     // The specification allows "implementation dependent" behavior
     // if an element on the prototype chain has an element that
     // might interact with sorting.
     max_prototype_element = CopyFromPrototype(this, length);
   }
 
-  var num_non_undefined = %IsObserved(this) ?
-      -1 : %RemoveArrayHoles(this, length);
+  // %RemoveArrayHoles returns -1 if fast removal is not supported.
+  var num_non_undefined = %RemoveArrayHoles(this, length);
 
   if (num_non_undefined == -1) {
     // The array is observed, or there were indexed accessors in the array.
     // Move array holes and undefineds to the end using a Javascript function
     // that is safe in the presence of accessors and is observable.
     num_non_undefined = SafeRemoveArrayHoles(this);
   }
 
   QuickSort(this, 0, num_non_undefined);
 
@@ skipping 505 matching lines @@
   ));
 
   SetUpLockedPrototype(InternalPackedArray, $Array(), $Array(
     "join", getFunction("join", ArrayJoin),
     "pop", getFunction("pop", ArrayPop),
     "push", getFunction("push", ArrayPush)
   ));
 }
 
 SetUpArray();