minor fix to get dartcombat tests passing

frog/lib/isolate.js

Index: frog/lib/isolate.js
diff --git a/frog/lib/isolate.js b/frog/lib/isolate.js
index 7061613c57f49de588451eade704eeef8615cac7..0d5c5fd290d6ceefc9ef3d811c37f6b94e381aaf 100644
--- a/frog/lib/isolate.js
+++ b/frog/lib/isolate.js
@@ -36,7 +36,7 @@ function isolate$receiveMessage(port, isolate,
   isolate$IsolateEvent.enqueue(isolate, function() {
     var message = isolate$deserializeMessage(serializedMessage);
     var replyTo = isolate$deserializeMessage(serializedReplyTo);
-    port._callback(message, replyTo);
+    if (port._callback) port._callback(message, replyTo);
   });
 }