Reland [Object.observe] Don't force normalization of elements for observed objects

src/builtins.cc

 // 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 293 matching lines @@
   if (array_proto->elements() != heap->empty_fixed_array()) return false;
   return array_proto->GetPrototype()->IsNull();
 }
 
 
 MUST_USE_RESULT
 static inline MaybeObject* EnsureJSArrayWithWritableFastElements(
     Heap* heap, Object* receiver, Arguments* args, int first_added_arg) {
   if (!receiver->IsJSArray()) return NULL;
   JSArray* array = JSArray::cast(receiver);
+  if (array->map()->is_observed()) return NULL;
   HeapObject* elms = array->elements();
   Map* map = elms->map();
   if (map == heap->fixed_array_map()) {
     if (args == NULL || array->HasFastObjectElements()) return elms;
   } else if (map == heap->fixed_cow_array_map()) {
     MaybeObject* maybe_writable_result = array->EnsureWritableFastElements();
     if (args == NULL || array->HasFastObjectElements() ||
         !maybe_writable_result->To(&elms)) {
       return maybe_writable_result;
     }
@@ skipping 1498 matching lines @@
 }
 BUILTIN_LIST_C(DEFINE_BUILTIN_ACCESSOR_C)
 BUILTIN_LIST_A(DEFINE_BUILTIN_ACCESSOR_A)
 BUILTIN_LIST_H(DEFINE_BUILTIN_ACCESSOR_H)
 BUILTIN_LIST_DEBUG_A(DEFINE_BUILTIN_ACCESSOR_A)
 #undef DEFINE_BUILTIN_ACCESSOR_C
 #undef DEFINE_BUILTIN_ACCESSOR_A
 
 
 } }  // namespace v8::internal