ES6 symbols: symbol properties should not cause going into slow mode.

src/objects.cc

 // Copyright 2013 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 1715 matching lines @@
                                        StoreFromKeyed store_mode) {
   ASSERT(!IsJSGlobalProxy());
   ASSERT(DescriptorArray::kNotFound ==
          map()->instance_descriptors()->Search(
              name, map()->NumberOfOwnDescriptors()));
 
   // Normalize the object if the name is an actual name (not the
   // hidden strings) and is not a real identifier.
   // Normalize the object if it will have too many fast properties.
   Isolate* isolate = GetHeap()->isolate();
-  if ((!IsIdentifier(isolate->unicode_cache(), name)
+  if ((!name->IsSymbol() && !IsIdentifier(isolate->unicode_cache(), name)
        && name != isolate->heap()->hidden_string()) ||
       (map()->unused_property_fields() == 0 &&
        TooManyFastProperties(properties()->length(), store_mode))) {
     Object* obj;
     MaybeObject* maybe_obj =
         NormalizeProperties(CLEAR_INOBJECT_PROPERTIES, 0);
     if (!maybe_obj->ToObject(&obj)) return maybe_obj;
 
     return AddSlowProperty(name, value, attributes);
   }
@@ skipping 12609 matching lines @@
   set_year(Smi::FromInt(year), SKIP_WRITE_BARRIER);
   set_month(Smi::FromInt(month), SKIP_WRITE_BARRIER);
   set_day(Smi::FromInt(day), SKIP_WRITE_BARRIER);
   set_weekday(Smi::FromInt(weekday), SKIP_WRITE_BARRIER);
   set_hour(Smi::FromInt(hour), SKIP_WRITE_BARRIER);
   set_min(Smi::FromInt(min), SKIP_WRITE_BARRIER);
   set_sec(Smi::FromInt(sec), SKIP_WRITE_BARRIER);
 }
 
 } }  // namespace v8::internal