After assignment return right hand side value instead of undefined when Object.isExtensible(o) === …

test/mjsunit/object-prevent-extensions.js

Index: test/mjsunit/object-prevent-extensions.js
===================================================================
--- test/mjsunit/object-prevent-extensions.js	(revision 10736)
+++ test/mjsunit/object-prevent-extensions.js	(working copy)
@@ -114,3 +114,16 @@
 
 foo.x = 29;
 assertEquals(undefined, foo.x);
+
+// http://es5.github.com/#x11.13.1
+// when Object.isExtensible(o) === false
+// assignment should return right hand side value
+var o = Object.preventExtensions({});
+var v = o.v = 50;
+assertEquals(undefined, o.v);
+assertEquals(50, v);
+
+// test same behavior as above, but for integer properties
+var n = o[0] = 100;
+assertEquals(undefined, o[0]);
+assertEquals(100, n);