Object.observe: include oldValue in change records,

src/property.h

 // 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 272 matching lines @@
         Object* value;
         value = holder()->property_dictionary()->ValueAt(GetDictionaryEntry());
         if (holder()->IsGlobalObject()) {
           value = JSGlobalPropertyCell::cast(value)->value();
         }
         return value;
       }
       case CONSTANT_FUNCTION:
         return GetConstantFunction();
       default:
-        return Smi::FromInt(0);
+        return Isolate::Current()->heap()->the_hole_value();
     }
   }
 
   Map* GetTransitionTarget() {
     ASSERT(IsTransition());
     TransitionArray* transitions = holder()->map()->transitions();
     return transitions->GetTarget(number_);
   }
 
   PropertyDetails GetTransitionDetails(Map* map) {
@@ skipping 104 matching lines @@
   JSReceiver* holder_;
   int number_;
   bool cacheable_;
   PropertyDetails details_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_PROPERTY_H_