Do not violate the assumption that fast-case arrays have Smi length...

src/objects.cc

 // Copyright 2006-2008 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 5142 matching lines @@
     JSArray* array = JSArray::cast(this);
     Object* return_value = array->JSArrayUpdateLengthFromIndex(index, value);
     if (return_value->IsFailure()) return return_value;
   }
 
   // Attempt to put this object back in fast case.
   if (ShouldConvertToFastElements()) {
     uint32_t new_length = 0;
     if (IsJSArray()) {
       CHECK(Array::IndexFromObject(JSArray::cast(this)->length(), &new_length));
+      JSArray::cast(this)->set_length(Smi::FromInt(new_length));
     } else {
       new_length = Dictionary::cast(elements())->max_number_key() + 1;
     }
     Object* obj = Heap::AllocateFixedArrayWithHoles(new_length);
     if (obj->IsFailure()) return obj;
     SetFastElements(FixedArray::cast(obj));
 #ifdef DEBUG
     if (FLAG_trace_normalization) {
       PrintF("Object elements are fast case again:\n");
       Print();
@@ skipping 1842 matching lines @@
   // No break point.
   if (break_point_objects()->IsUndefined()) return 0;
   // Single beak point.
   if (!break_point_objects()->IsFixedArray()) return 1;
   // Multiple break points.
   return FixedArray::cast(break_point_objects())->length();
 }
 
 
 } }  // namespace v8::internal