Add ES5 Object.isExtensible and Object.preventExtensions.

src/messages.js

Index: src/messages.js
===================================================================
--- src/messages.js	(revision 5007)
+++ src/messages.js	(working copy)
@@ -196,6 +196,7 @@
       circular_structure:           "Converting circular structure to JSON",
       obj_ctor_property_non_object: "Object.%0 called on non-object",
       array_indexof_not_defined:    "Array.getIndexOf: Argument undefined",
+      object_not_extensible:        "Can't add property %0, object is not extensible",
       illegal_access:               "illegal access"
     };
   }