Re-apply "Adding a mechanism to 'upgrade' the Dart type of elements"

tools/dom/src/shared_html.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 dart.dom.html;
 
 _wrapZone(callback(arg)) {
   // For performance reasons avoid wrapping if we are in the root zone.
   if (Zone.current == Zone.ROOT) return callback;
   if (callback == null) return null;
@@ skipping 50 matching lines @@
  * save typing a few characters.
  *
  * [selectors] should be a string using CSS selector syntax.
  *
  *     var items = document.querySelectorAll('.itemClassName');
  *
  * For details about CSS selector syntax, see the
  * [CSS selector specification](http://www.w3.org/TR/css3-selectors/).
  */
 ElementList querySelectorAll(String selectors) => document.querySelectorAll(selectors);
+
+/// A utility for changing the Dart wrapper type for elements.
+abstract class ElementUpgrader {
+  /// Upgrade the specified element to be of the Dart type this was created for.
+  ///
+  /// After upgrading the element passed in is invalid and the returned value
+  /// should be used instead.
+  Element upgrade(Element element);
+}