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

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 0afc0372d938e119527cb7f6173073b2d8b2b747..f39d59c28ad742026690bb3f20a83337999cdff2 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -1,4 +1,4 @@
-// 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:
@@ -769,8 +769,7 @@ function DefineOneShotAccessor(obj, name, fun) {
     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) {