Make the runtime entry for setting/changing accessors "atomic".

src/messages.js

-// Copyright 2011 the V8 project authors. All rights reserved.
+// 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
 //       with the distribution.
@@ skipping 750 matching lines @@
       return value;
     }
     hasBeenSet = true;
     value = fun(obj);
     return value;
   };
   var setter = function(v) {
     hasBeenSet = true;
     value = v;
   };
-  %DefineOrRedefineAccessorProperty(obj, name, GETTER, getter, DONT_ENUM);
-  %DefineOrRedefineAccessorProperty(obj, name, SETTER, setter, DONT_ENUM);
+  %DefineOrRedefineAccessorProperty(obj, name, getter, setter, DONT_ENUM);
 }
 
 function CallSite(receiver, fun, pos) {
   this.receiver = receiver;
   this.fun = fun;
   this.pos = pos;
 }
 
 function CallSiteGetThis() {
   return this.receiver;
@@ skipping 431 matching lines @@
     throw e;
   }
 }
 
 
 InstallFunctions($Error.prototype, DONT_ENUM, ['toString', ErrorToString]);
 
 // Boilerplate for exceptions for stack overflows. Used from
 // Isolate::StackOverflow().
 var kStackOverflowBoilerplate = MakeRangeError('stack_overflow', []);