Elide write barrier.

src/objects.cc

 // Copyright 2011 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 908 matching lines @@
         String* first = cs->first();
         int first_length = first->length();
         WriteToFlat(first, dest, 0, first_length);
         String* second = cs->second();
         WriteToFlat(second,
                     dest + first_length,
                     0,
                     len - first_length);
       }
       cs->set_first(result);
-      cs->set_second(heap->empty_string());
+      cs->set_second(heap->empty_string(), SKIP_WRITE_BARRIER);
       return result;
     }
     default:
       return this;
   }
 }
 
 
 bool String::MakeExternal(v8::String::ExternalStringResource* resource) {
   // Externalizing twice leaks the external resource, so it's
@@ skipping 11572 matching lines @@
   if (break_point_objects()->IsUndefined()) return 0;
   // Single break point.
   if (!break_point_objects()->IsFixedArray()) return 1;
   // Multiple break points.
   return FixedArray::cast(break_point_objects())->length();
 }
 #endif  // ENABLE_DEBUGGER_SUPPORT
 
 
 } }  // namespace v8::internal