A JSArray may have a filler map in the elements pointer.

src/objects-debug.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 246 matching lines @@
 
 template <class Traits>
 void FixedTypedArray<Traits>::FixedTypedArrayVerify() {
   CHECK(IsHeapObject() &&
         HeapObject::cast(this)->map()->instance_type() ==
             Traits::kInstanceType);
 }
 
 
 bool JSObject::ElementsAreSafeToExamine() {
-  return (FLAG_use_gvn && FLAG_use_allocation_folding) ||
-      reinterpret_cast<Map*>(elements()) !=
+  // If a GC was caused while constructing this object, the elements
+  // pointer may point to a one pointer filler map.
+  return reinterpret_cast<Map*>(elements()) !=
       GetHeap()->one_pointer_filler_map();
 }
 
 
 void JSObject::JSObjectVerify() {
   VerifyHeapPointer(properties());
   VerifyHeapPointer(elements());
 
   if (GetElementsKind() == NON_STRICT_ARGUMENTS_ELEMENTS) {
     CHECK(this->elements()->IsFixedArray());
@@ skipping 871 matching lines @@
   for (int i = 0; i < number_of_transitions(); ++i) {
     if (!CheckOneBackPointer(current_map, GetTarget(i))) return false;
   }
   return true;
 }
 
 
 #endif  // DEBUG
 
 } }  // namespace v8::internal