Do not cache functions until we know they are fully constructed.

src/execution.cc

Index: src/execution.cc
===================================================================
--- src/execution.cc	(revision 1050)
+++ src/execution.cc	(working copy)
@@ -439,7 +439,7 @@
   int serial_number = Smi::cast(data->serial_number())->value();
   Object* elm =
       Top::global_context()->function_cache()->GetElement(serial_number);
-  if (!elm->IsUndefined()) return Handle<JSFunction>(JSFunction::cast(elm));
+  if (elm->IsJSFunction()) return Handle<JSFunction>(JSFunction::cast(elm));
   // The function has not yet been instantiated in this context; do it.
   Object** args[1] = { Handle<Object>::cast(data).location() };
   Handle<Object> result =