work around for https://github.com/Polymer/TemplateBinding/issues/150

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 727 matching lines @@
   /**
    * Bind events via attributes of the form `on-eventName`. This method can be
    * use to hooks into the model syntax and adds event listeners as needed. By
    * default, binding paths are always method names on the root model, the
    * custom element in which the node exists. Adding a '@' in the path directs
    * the event binding to use the model path as the event listener.  In both
    * cases, the actual listener is attached to a generic method which evaluates
    * the bound path at event execution time.
    */
   // from src/instance/event.js#prepareBinding
-  // Dart note: template_binding doesn't have the notion of prepareBinding, so
-  // we implement this by wrapping/overriding getBinding instead.
   // TODO(sorvell): we're patching the syntax while evaluating
   // event bindings. we'll move this to a better spot when that's done
   static PrepareBindingFunction prepareBinding(String path, String name, node,
       originalPrepareBinding) {
 
     // if lhs an event prefix,
     if (!_hasEventPrefix(name)) return originalPrepareBinding(path, name, node);
 
     // provide an event-binding callback.
     return (model, node) {
@@ skipping 330 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);
 }