Move function serialization to sync ports.

lib/isolate/serialization.dart

Index: lib/isolate/serialization.dart
diff --git a/lib/isolate/serialization.dart b/lib/isolate/serialization.dart
index 4eef8a3f0b7746690c52b522d4fbd8648a1fe6d9..b3cb136eafa562f9702efc417bc01966d5bc5d3a 100644
--- a/lib/isolate/serialization.dart
+++ b/lib/isolate/serialization.dart
@@ -37,6 +37,7 @@ class _MessageTraverser {
     if (x is Map) return visitMap(x);
     if (x is SendPort) return visitSendPort(x);
     if (x is SendPortSync) return visitSendPortSync(x);
+    if (x is Function) return visitFunction(x);
 
     // TODO(floitsch): make this a real exception. (which one)?
     throw "Message serialization: Illegal value $x passed";
@@ -48,6 +49,10 @@ class _MessageTraverser {
   abstract visitSendPort(SendPort x);
   abstract visitSendPortSync(SendPortSync x);
 
+  visitFunction(Function func) {
+    throw "Serialization of functions is not allowed.";
+  }
+
   static bool isPrimitive(x) {
     return (x === null) || (x is String) || (x is num) || (x is bool);
   }