fix binding to $ from polymer templates

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 86 matching lines @@
    * Shadow roots created by [parseElement]. See [getShadowRoot].
    */
   final _shadowRoots = new HashMap<String, ShadowRoot>();
 
   /** Map of items in the shadow root(s) by their [Element.id]. */
   // TODO(jmesserly): various issues:
   // * wrap in UnmodifiableMapView?
   // * should we have an object that implements noSuchMethod?
   // * should the map have a key order (e.g. LinkedHash or SplayTree)?
   // * should this be a live list? Polymer doesn't, maybe due to JS limitations?
-  // For now I picked the most performant choice: non-live HashMap.
-  final Map<String, Element> $ = new HashMap<String, Element>();
+  // Note: this is observable to support $['someId'] being used in templates.
+  // The template is stamped before $ is populated, so we need observation if
+  // we want it to be usable in bindings.
+  @reflectable final Map<String, Element> $ =
+      new ObservableMap<String, Element>();
 
   /**
    * Gets the shadow root associated with the corresponding custom element.
    *
    * This is identical to [shadowRoot], unless there are multiple levels of
    * inheritance and they each have their own shadow root. For example,
    * this can happen if the base class and subclass both have `<template>` tags
    * in their `<polymer-element>` tags.
    */
   // TODO(jmesserly): Polymer does not have this feature. Reconcile.
@@ skipping 1016 matching lines @@
 class PolymerElement extends HtmlElement with Polymer, Observable {
   PolymerElement.created() : super.created() {
     polymerCreated();
   }
 }
 
 class _PropertyValue {
   Object oldValue, newValue;
   _PropertyValue(this.oldValue);
 }