Convert most of the javascript isolate code into Dart, inject JS code only when

frog/lib/isolate_serialization.dart

Index: frog/lib/isolate_serialization.dart
diff --git a/frog/lib/isolate_serialization.dart b/frog/lib/isolate_serialization.dart
index d836e2f0a5d66ea88e848be7bf125948c8396382..2ab10a3b45d0d2303f005c98e31c3a84c66db48d 100644
--- a/frog/lib/isolate_serialization.dart
+++ b/frog/lib/isolate_serialization.dart
@@ -117,11 +117,11 @@ class Copier extends MessageTraverser {
   }
 
   SendPort visitReceivePort(ReceivePortImpl port) {
-    return port._toNewSendPort();
+    return port.toSendPort();
   }
 
   SendPort visitReceivePortSingleShot(ReceivePortSingleShotImpl port) {
-    return port._toNewSendPort();
+    return port.toSendPort();
   }
 }