ES6 symbols: Allow symbols as property names

src/json-stringifier.h

 // 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 522 matching lines @@
 
   Append('{');
   bool comma = false;
 
   if (object->HasFastProperties() &&
       !object->HasIndexedInterceptor() &&
       !object->HasNamedInterceptor() &&
       object->elements()->length() == 0) {
     Handle<Map> map(object->map());
     for (int i = 0; i < map->NumberOfOwnDescriptors(); i++) {
-      Handle<String> key(map->instance_descriptors()->GetKey(i), isolate_);
+      Handle<Name> name(map->instance_descriptors()->GetKey(i), isolate_);
+      // TODO(rossberg): Should this throw?
+      if (!name->IsString()) continue;
+      Handle<String> key = Handle<String>::cast(name);
       PropertyDetails details = map->instance_descriptors()->GetDetails(i);
       if (details.IsDontEnum() || details.IsDeleted()) continue;
       Handle<Object> property;
       if (details.type() == FIELD && *map == object->map()) {
         property = Handle<Object>(
                        object->FastPropertyAt(
                            map->instance_descriptors()->GetFieldIndex(i)),
                        isolate_);
       } else {
         property = GetProperty(object, key);
@@ skipping 182 matching lines @@
       SerializeString_<false, uint8_t>(object);
     } else {
       SerializeString_<false, uc16>(object);
     }
   }
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_STRINGIFIER_H_