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

src/builtins.cc

Index: src/builtins.cc
diff --git a/src/builtins.cc b/src/builtins.cc
index b614904c9f468ba08708c8a3bb96710598415ab2..6040e17c9d3ca081ddec863ac65bab027fd4bdfd 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -311,6 +311,7 @@ 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()) {