Follow jsc on not throwing when trying to add a property to a non-extensible object.

src/objects.h

Index: src/objects.h
===================================================================
--- src/objects.h	(revision 7276)
+++ src/objects.h	(working copy)
@@ -1691,7 +1691,8 @@
   // Add a property to an object.
   MUST_USE_RESULT MaybeObject* AddProperty(String* name,
                                            Object* value,
-                                           PropertyAttributes attributes);
+                                           PropertyAttributes attributes,
+                                           StrictModeFlag strict_mode);
 
   // Convert the object to use the canonical dictionary
   // representation. If the object is expected to have additional properties