Object.observe: prevent observed objects from using fast elements.

src/objects-inl.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 1408 matching lines @@
   Object* obj;
   ElementsKind elements_kind = GetInitialFastElementsKind();
   if (!FLAG_smi_only_arrays) {
     elements_kind = FastSmiToObjectElementsKind(elements_kind);
   }
   MaybeObject* maybe_obj = GetElementsTransitionMap(GetIsolate(),
                                                     elements_kind);
   if (!maybe_obj->ToObject(&obj)) return maybe_obj;
   set_map(Map::cast(obj));
   initialize_elements();
+  if (FLAG_harmony_observation && map()->is_observed()) {
+    // Maintain invariant that observed elements are always in dictionary mode.
+    // For this to work on arrays, we have to make sure to reset length first.
+    if (IsJSArray()) JSArray::cast(this)->set_length(Smi::FromInt(0));
+    maybe_obj = NormalizeElements();
+    if (maybe_obj->IsFailure()) return maybe_obj;
+  }
   return this;
 }
 
 
 MaybeObject* JSObject::AddFastPropertyUsingMap(Map* map) {
   ASSERT(this->map()->NumberOfOwnDescriptors() + 1 ==
          map->NumberOfOwnDescriptors());
   if (this->map()->unused_property_fields() == 0) {
     int new_size = properties()->length() + map->unused_property_fields() + 1;
     FixedArray* new_properties;
@@ skipping 4300 matching lines @@
 #undef WRITE_UINT32_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_