port polymer data binding tests

pkg/custom_element/lib/polyfill.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.
 
 /** Dart APIs for interacting with the JavaScript Custom Elements polyfill. */
 library custom_element.polyfill;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:js' as js;
@@ skipping 24 matching lines @@
 
   var customElements = js.context['CustomElements'];
   if (customElements == null) {
     // Return true if native document.register, otherwise false.
     // (Maybe the polyfill isn't loaded yet. Wait for it.)
     return document.supportsRegister;
   }
 
   return customElements['ready'] == true;
 }
+
+/**
+ * Loads `custom-elements.debug.js` or `custom-elements.min.js` by adding the
+ * script tag to the page. Returns a future that completes when custom elements
+ * are ready (equivalent to [customElementsReady]).
+ *
+ * Normally you should add this to your HTML file
+ * (the Polymer package will do this automatically), but loading dynamically
+ * can be useful for scenarios such as tests.
+ */
+Future loadCustomElementPolyfill() {
+  if (!document.supportsRegister && !js.context.hasProperty('CustomElements')) {
+    var script = new ScriptElement()
+        ..src = '/packages/custom_element/custom-elements.debug.js';
+    document.head.append(script);
+    return document.on['WebComponentsReady'].first;
+  }
+  return new Future.value();
+}