Fix Array.prototype.push and Array.prototype.unshift for read-only length.

src/array.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 "use strict";
 
 // This file relies on the fact that the following declarations have been made
 // in runtime.js:
 // var $Array = global.Array;
 
@@ skipping 610 matching lines @@
   CHECK_OBJECT_COERCIBLE(this, "Array.prototype.unshift");
 
   if (%IsObserved(this))
     return ObservedArrayUnshift.apply(this, arguments);
 
   var array = TO_OBJECT_INLINE(this);
   var len = TO_UINT32(array.length);
   var num_arguments = %_ArgumentsLength();
   var is_sealed = ObjectIsSealed(array);
 
-  if (IS_ARRAY(array) && !is_sealed) {
+  if (IS_ARRAY(array) && !is_sealed && len > 0) {
     SmartMove(array, 0, 0, len, num_arguments);
   } else {
     SimpleMove(array, 0, 0, len, num_arguments);
   }
 
   for (var i = 0; i < num_arguments; i++) {
     array[i] = %_Arguments(i);
   }
 
   var new_length = len + num_arguments;
@@ skipping 883 matching lines @@
   ));
 
   SetUpLockedPrototype(InternalPackedArray, $Array(), $Array(
     "join", getFunction("join", ArrayJoin),
     "pop", getFunction("pop", ArrayPop),
     "push", getFunction("push", ArrayPush)
   ));
 }
 
 SetUpArray();