Adding polyfill support for custom elements.

tools/dom/src/dartium_Platform.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 html;
 
 class Platform {
   /**
    * Returns true if dart:typed_data types are supported on this
    * browser.  If false, using these types will generate a runtime
    * error.
    */
   static final supportsTypedData = true;
 
   /**
    * Returns true if SIMD types in dart:typed_data types are supported
    * on this browser.  If false, using these types will generate a runtime
    * error.
    */
   static final supportsSimd = true;
+
+  /**
+   * Upgrade all custom elements in the subtree which have not been upgraded.
+   *
+   * This is needed to cover timing scenarios which the custom element polyfill
+   * does not cover.
+   */
+  void upgradeCustomElements(Node node) {
+    // no-op, provided for dart2js polyfill.
+  }
 }