Add access checks to Object.preventExtensions + add regression test for 1027....

src/runtime.cc

Index: src/runtime.cc
===================================================================
--- src/runtime.cc	(revision 6829)
+++ src/runtime.cc	(working copy)
@@ -3673,6 +3673,8 @@
       is_element) {
     // Normalize the elements to enable attributes on the property.
     if (js_object->IsJSGlobalProxy()) {
+      // We do not need to do access checks here since these has already
+      // been performed by the call to GetOwnProperty.
       Handle<Object> proto(js_object->GetPrototype());
       // If proxy is detached, ignore the assignment. Alternatively,
       // we could throw an exception.