Add support for JS->Dart and Dart->dart via *PortSync.

lib/isolate/serialization.dart

Index: lib/isolate/serialization.dart
diff --git a/lib/isolate/serialization.dart b/lib/isolate/serialization.dart
index 11e9ad9f43c16403a7e87b8a7b7f315ae958e22f..4eef8a3f0b7746690c52b522d4fbd8648a1fe6d9 100644
--- a/lib/isolate/serialization.dart
+++ b/lib/isolate/serialization.dart
@@ -36,6 +36,7 @@ class _MessageTraverser {
     if (x is List) return visitList(x);
     if (x is Map) return visitMap(x);
     if (x is SendPort) return visitSendPort(x);
+    if (x is SendPortSync) return visitSendPortSync(x);
 
     // TODO(floitsch): make this a real exception. (which one)?
     throw "Message serialization: Illegal value $x passed";
@@ -45,6 +46,7 @@ class _MessageTraverser {
   abstract visitList(List x);
   abstract visitMap(Map x);
   abstract visitSendPort(SendPort x);
+  abstract visitSendPortSync(SendPortSync x);
 
   static bool isPrimitive(x) {
     return (x === null) || (x is String) || (x is num) || (x is bool);