Allow objects with "" properties to stay fast.

src/heap.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 3289 matching lines @@
   { MaybeObject* maybe_obj = AllocateSymbol();
     if (!maybe_obj->ToObject(&obj)) return false;
   }
   Symbol::cast(obj)->set_is_private(true);
   set_frozen_symbol(Symbol::cast(obj));
 
   { MaybeObject* maybe_obj = AllocateSymbol();
     if (!maybe_obj->ToObject(&obj)) return false;
   }
   Symbol::cast(obj)->set_is_private(true);
+  set_nonexistent_symbol(Symbol::cast(obj));
+
+  { MaybeObject* maybe_obj = AllocateSymbol();
+    if (!maybe_obj->ToObject(&obj)) return false;
+  }
+  Symbol::cast(obj)->set_is_private(true);
   set_elements_transition_symbol(Symbol::cast(obj));
 
   { MaybeObject* maybe_obj = SeededNumberDictionary::Allocate(this, 0, TENURED);
     if (!maybe_obj->ToObject(&obj)) return false;
   }
   SeededNumberDictionary::cast(obj)->set_requires_slow_elements();
   set_empty_slow_element_dictionary(SeededNumberDictionary::cast(obj));
 
   { MaybeObject* maybe_obj = AllocateSymbol();
     if (!maybe_obj->ToObject(&obj)) return false;
@@ skipping 4423 matching lines @@
       static_cast<int>(object_sizes_last_time_[index]));
   CODE_AGE_LIST_COMPLETE(ADJUST_LAST_TIME_OBJECT_COUNT)
 #undef ADJUST_LAST_TIME_OBJECT_COUNT
 
   OS::MemCopy(object_counts_last_time_, object_counts_, sizeof(object_counts_));
   OS::MemCopy(object_sizes_last_time_, object_sizes_, sizeof(object_sizes_));
   ClearObjectStats();
 }
 
 } }  // namespace v8::internal