Make __proto__ a foreign callback on Object.prototype.

src/json-parser.h

 // Copyright 2011 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 272 matching lines @@
     }
     return ReportUnexpectedCharacter();
   }
   return ReportUnexpectedCharacter();
 }
 
 
 // Parse a JSON object. Position must be right at '{'.
 template <bool seq_ascii>
 Handle<Object> JsonParser<seq_ascii>::ParseJsonObject() {
-  Handle<Object> prototype;
   Handle<JSObject> json_object =
       factory()->NewJSObject(object_constructor(), pretenure_);
   ASSERT_EQ(c0_, '{');
 
   AdvanceSkipWhitespace();
   if (c0_ != '}') {
     do {
       if (c0_ != '"') return ReportUnexpectedCharacter();
 
       int start_position = position_;
@@ skipping 34 matching lines @@
       c0_ = '"';
 #endif
 
       Handle<String> key = ParseJsonSymbol();
       if (key.is_null() || c0_ != ':') return ReportUnexpectedCharacter();
 
       AdvanceSkipWhitespace();
       Handle<Object> value = ParseJsonValue();
       if (value.is_null()) return ReportUnexpectedCharacter();
 
-      if (key->Equals(isolate()->heap()->Proto_symbol())) {
-        prototype = value;
+      if (JSObject::TryTransitionToField(json_object, key)) {
+        int index = json_object->LastAddedFieldIndex();
+        json_object->FastPropertyAtPut(index, *value);
       } else {
-        if (JSObject::TryTransitionToField(json_object, key)) {
-          int index = json_object->LastAddedFieldIndex();
-          json_object->FastPropertyAtPut(index, *value);
-        } else {
-          JSObject::SetLocalPropertyIgnoreAttributes(
-              json_object, key, value, NONE);
-        }
+        JSObject::SetLocalPropertyIgnoreAttributes(
+            json_object, key, value, NONE);
       }
     } while (MatchSkipWhiteSpace(','));
     if (c0_ != '}') {
       return ReportUnexpectedCharacter();
     }
-    if (!prototype.is_null()) SetPrototype(json_object, prototype);
   }
   AdvanceSkipWhitespace();
   return json_object;
 }
 
 // Parse a JSON array. Position must be right at '['.
 template <bool seq_ascii>
 Handle<Object> JsonParser<seq_ascii>::ParseJsonArray() {
   ZoneScope zone_scope(zone(), DELETE_ON_EXIT);
   ZoneList<Handle<Object> > elements(4, zone());
@@ skipping 330 matching lines @@
   }
   ASSERT_EQ('"', c0_);
   // Advance past the last '"'.
   AdvanceSkipWhitespace();
   return result;
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_PARSER_H_