Remove signature classes from the VM.

runtime/vm/message.cc

Index: runtime/vm/message.cc
diff --git a/runtime/vm/message.cc b/runtime/vm/message.cc
index 54f40b4eeb77c91cee4a35bbd2c5d356a0e75b48..195b2c6c81bd9abddc8e87f894d3b04f8655ec18 100644
--- a/runtime/vm/message.cc
+++ b/runtime/vm/message.cc
@@ -203,9 +203,9 @@ void MessageQueue::PrintJSON(JSONStream* stream) {
     // TODO(johnmccutchan): Move port -> handler map out of Dart and into the
     // VM, that way we can lookup the handler without invoking Dart code.
     msg_handler = DartLibraryCalls::LookupHandler(current->dest_port());
-    if (msg_handler.IsInstance() && Instance::Cast(msg_handler).IsClosure()) {
+    if (msg_handler.IsClosure()) {
       // Grab function from closure.
-      msg_handler = Closure::function(Instance::Cast(msg_handler));
+      msg_handler = Closure::Cast(msg_handler).function();
     }
     if (msg_handler.IsFunction()) {
       const Function& function = Function::Cast(msg_handler);