Implement correct checking for inherited readonliness on assignment.

test/mjsunit/with-readonly.js

 // Copyright 2010 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 18 matching lines @@
 
 // Create an object with a read-only length property in the prototype
 // chain by putting the string split function in the prototype chain.
 var o = {};
 o.__proto__ = String.prototype.split;
 
 function f() {
   with (o) {
     length = 23;
     length = 24;
-    assertEquals(24, length);
+    assertEquals(2, length);
   }
+  assertEquals(2, o.length);
 }
 f();
-