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

lib/dom/templates/html/frog/impl_Window.darttemplate

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class $CLASSNAME$EXTENDS$IMPLEMENTS native "@*DOMWindow" {
 
   _DocumentImpl get document() native "return this.document;";
 
   Window get _top() native "return this.top;";
 
@@ skipping 100 matching lines @@
    };
    this.cancelAnimationFrame = function(id) { clearTimeout(id); }
 ''';
 
 
   _IDBFactoryImpl get indexedDB() => _get_indexedDB();
 
   _IDBFactoryImpl _get_indexedDB() native
       'return this.indexedDB || this.webkitIndexedDB || this.mozIndexedDB';
 
-  // TODO(kasperl): Document this.
+  // TODO(kasperl): Document these.
   lookupPort(String name) {
     var port = JSON.parse(localStorage['dart-port:$name']);
     return _deserialize(port);
   }
 
+  registerPort(String name, var port) {
+    var serialized = _serialize(port);
+    localStorage['dart-port:$name'] = JSON.stringify(serialized);
+  }
+
 $!MEMBERS
 }