Fix bug 6713

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
===================================================================
--- runtime/vm/parser.cc	(revision 15110)
+++ runtime/vm/parser.cc	(working copy)
@@ -5035,10 +5035,14 @@
   // class in the current library (but not in its imports) and only create a new
   // canonical signature class if it does not exist yet.
   const String& signature = String::Handle(function.Signature());
-  Class& signature_class = Class::ZoneHandle(
-      library_.LookupLocalClass(signature));
-
+  Class& signature_class = Class::ZoneHandle();
+  if (!is_new_closure) {
+    signature_class = function.signature_class();
+  }
   if (signature_class.IsNull()) {
+    signature_class = library_.LookupLocalClass(signature);
+  }
+  if (signature_class.IsNull()) {
     // If we don't have a signature class yet, this must be a closure we
     // have not parsed before.
     ASSERT(is_new_closure);