Make sure that we do not call is_extensible on the global proxy....

src/runtime.cc

 // 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 866 matching lines @@
   return *desc;
 }
 
 
 static MaybeObject* Runtime_PreventExtensions(Arguments args) {
   ASSERT(args.length() == 1);
   CONVERT_CHECKED(JSObject, obj, args[0]);
   return obj->PreventExtensions();
 }
 
+
 static MaybeObject* Runtime_IsExtensible(Arguments args) {
   ASSERT(args.length() == 1);
   CONVERT_CHECKED(JSObject, obj, args[0]);
-  return obj->map()->is_extensible() ?  Heap::true_value()
+  if (obj->IsJSGlobalProxy()) {
+    Object* proto = obj->GetPrototype();
+    if (proto->IsNull()) return Heap::false_value();
+    ASSERT(proto->IsJSGlobalObject());
+    obj = JSObject::cast(proto);
+  }
+  return obj->map()->is_extensible() ? Heap::true_value()
                                      : Heap::false_value();
 }
 
 
 static MaybeObject* Runtime_RegExpCompile(Arguments args) {
   HandleScope scope;
   ASSERT(args.length() == 3);
   CONVERT_ARG_CHECKED(JSRegExp, re, 0);
   CONVERT_ARG_CHECKED(String, pattern, 1);
   CONVERT_ARG_CHECKED(String, flags, 2);
@@ skipping 10166 matching lines @@
   } else {
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     Counters::gc_last_resort_from_js.Increment();
     Heap::CollectAllGarbage(false);
   }
 }
 
 
 } }  // namespace v8::internal