Make __proto__ a real JavaScript accessor property.

src/messages.js

 // 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 80 matching lines @@
   proxy_prop_not_configurable:   ["Proxy handler ", "%0", " returned non-configurable descriptor for property '", "%2", "' from '", "%1", "' trap"],
   proxy_non_object_prop_names:   ["Trap '", "%1", "' returned non-object ", "%0"],
   proxy_repeated_prop_name:      ["Trap '", "%1", "' returned repeated property name '", "%2", "'"],
   invalid_weakmap_key:           ["Invalid value used as weak map key"],
   not_date_object:               ["this is not a Date object."],
   observe_non_object:            ["Object.", "%0", " cannot ", "%0", " non-object"],
   observe_non_function:          ["Object.", "%0", " cannot deliver to non-function"],
   observe_callback_frozen:       ["Object.observe cannot deliver to a frozen function object"],
   observe_type_non_string:       ["Invalid changeRecord with non-string 'type' property"],
   observe_notify_non_notifier:   ["notify called on non-notifier object"],
+  proto_poison_pill:             ["Generic use of __proto__ accessor not allowed"],
   // RangeError
   invalid_array_length:          ["Invalid array length"],
   invalid_array_buffer_length:   ["Invalid array buffer length"],
   stack_overflow:                ["Maximum call stack size exceeded"],
   invalid_time_value:            ["Invalid time value"],
   // SyntaxError
   unable_to_parse:               ["Parse error"],
   invalid_regexp_flags:          ["Invalid flags supplied to RegExp constructor '", "%0", "'"],
   invalid_regexp:                ["Invalid RegExp pattern /", "%0", "/"],
   illegal_break:                 ["Illegal break statement"],
@@ skipping 1197 matching lines @@
     %SetOverflowedStackTrace(this, void 0);
   }
 
   %DefineOrRedefineAccessorProperty(
       boilerplate, 'stack', getter, setter, DONT_ENUM);
 
   return boilerplate;
 }
 
 var kStackOverflowBoilerplate = SetUpStackOverflowBoilerplate();