Fix copy-on-write assert by setting the new array map early.

src/objects-inl.h

 // 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 3169 matching lines @@
   bool result = elements()->IsFixedArray();
   ASSERT(result == (!HasPixelElements() && !HasExternalArrayElements()));
   return result;
 }
 
 
 Object* JSObject::EnsureWritableFastElements() {
   ASSERT(HasFastElements());
   FixedArray* elems = FixedArray::cast(elements());
   if (elems->map() != Heap::fixed_cow_array_map()) return elems;
-  Object* writable_elems = Heap::CopyFixedArray(elems);
+  Object* writable_elems = Heap::CopyFixedArrayWithMap(elems,
+                                                       Heap::fixed_array_map());
   if (writable_elems->IsFailure()) return writable_elems;
-  FixedArray::cast(writable_elems)->set_map(Heap::fixed_array_map());
   set_elements(FixedArray::cast(writable_elems));
   Counters::cow_arrays_converted.Increment();
   return writable_elems;
 }
 
 
 StringDictionary* JSObject::property_dictionary() {
   ASSERT(!HasFastProperties());
   return StringDictionary::cast(properties());
 }
@@ skipping 335 matching lines @@
 #undef WRITE_INT_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_