workaround for event handlers: don't return StreamSubscription as model

pkg/polymer/lib/src/instance.dart

 // Copyright (c) 2013, 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.
 
 part of polymer;
 
 /**
  * Use this annotation to publish a field as an attribute. For example:
  *
  *     class MyPlaybackElement extends PolymerElement {
@@ skipping 745 matching lines @@
     // provide an event-binding callback.
     return (model, node) {
       if (_eventsLog.isLoggable(Level.FINE)) {
         _eventsLog.fine('event: [$node].$name => [$model].$path())');
       }
       var eventName = _removeEventPrefix(name);
       // TODO(sigmund): polymer.js dropped event translations. reconcile?
       var translated = _eventTranslations[eventName];
       eventName = translated != null ? translated : eventName;
 
-      // TODO(jmesserly): returning a StreamSubscription as the model is quite
-      // strange. package:template_binding doesn't have any cleanup logic to
-      // handle that.
-      return node.on[eventName].listen((event) {
+      // TODO(jmesserly): we need a place to unregister this. See:
+      // https://code.google.com/p/dart/issues/detail?id=15574
+      node.on[eventName].listen((event) {
         var ctrlr = _findController(node);
         if (ctrlr is! Polymer) return;
         var obj = ctrlr;
         var method = path;
         if (path[0] == '@') {
           obj = model;
           method = new PathObserver(model, path.substring(1)).value;
         }
         var detail = event is CustomEvent ?
             (event as CustomEvent).detail : null;
         ctrlr.dispatchMethod(obj, method, [event, detail, node]);
       });
+
+      // TODO(jmesserly): this return value is bogus. Returning null here causes
+      // the wrong thing to happen in template_binding.
+      return new ObservableBox();
     };
   }
 
   // TODO(jmesserly): this won't find the correct host unless the ShadowRoot
   // was created on a PolymerElement.
   static Polymer _findController(Node node) {
     while (node.parentNode != null) {
       node = node.parentNode;
     }
     return _shadowHost[node];
@@ skipping 296 matching lines @@
 
 class _PropertyValue {
   Object oldValue, newValue;
   _PropertyValue(this.oldValue);
 }
 
 class _PolymerExpressionsWithEventDelegate extends PolymerExpressions {
   prepareBinding(String path, name, node) =>
       Polymer.prepareBinding(path, name, node, super.prepareBinding);
 }