Make sure that we never call prevent extension on the global proxy,...

src/objects.cc

Index: src/objects.cc
===================================================================
--- src/objects.cc	(revision 6673)
+++ src/objects.cc	(working copy)
@@ -2779,6 +2779,13 @@
 
 
 MaybeObject* JSObject::PreventExtensions() {
+  if (IsJSGlobalProxy()) {
+    Object* proto = GetPrototype();
+    if (proto->IsNull()) return this;
+    ASSERT(proto->IsJSGlobalObject());
+    return JSObject::cast(proto)->PreventExtensions();
+  }
+
   // If there are fast elements we normalize.
   if (HasFastElements()) {
     Object* ok;