fix auto-binding-dart example

pkg/polymer/lib/auto_binding.dart

 // Copyright (c) 2014, 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.
 
 library polymer.auto_binding;
 
 import 'dart:html';
 import 'package:polymer/polymer.dart';
 import 'package:template_binding/template_binding.dart';
 
 /**
  * The `auto-binding-dart` element extends the template element. It provides a
  * quick and easy way to do data binding without the need to setup a binding
  * delegate or use the [templateBind] call. Both data and event handlers can be
  * bound using the [model].
  *
  * The `auto-binding-dart` element acts just like a template that is bound to
  * a model. It stamps its content in the dom adjacent to itself. When the
  * content is stamped, the `template-bound` event is fired.
  *
+ * **NOTE**: It is a good idea to use an id to select these elements. During
+ * code transformation it is likely that other template elements will be
+ * inserted at the top of the body above yours, so something like
+ * querySelector('template') is likely to break when built.
+ * See http://dartbug.com/20911.
+ *
  * Example:
  *
- *     <template is="auto-binding-dart">
+ *     <template id="my-template" is="auto-binding-dart">
  *       <div>Say something: <input value="{{value}}"></div>
  *       <div>You said: {{value}}</div>
  *       <button on-tap="{{buttonTap}}">Tap me!</button>
  *     </template>
  *     <script type="application/dart">
  *       import 'dart:html';
  *       import 'package:polymer/polymer.dart';
  *
  *       main() {
- *         var template = document.querySelector('template');
+ *         var template = document.querySelector('#my-template');
  *         template.model = new MyModel();
  *       }
  *
  *       class MyModel {
  *         String value = 'something';
  *         buttonTap() => console.log('tap!');
  *       }
  *     </script>
  *
  */
@@ skipping 54 matching lines @@
   }
 }
 
 // Dart note: this is implemented a little differently to keep it in classic
 // OOP style. Instead of monkeypatching findController, override it.
 class _AutoBindingSyntax extends PolymerExpressions {
   final AutoBindingElement _node;
   _AutoBindingSyntax(this._node) : super();
   @override findController(_) => _node;
 }