Revert "Adding a mechanism to 'upgrade' the Dart type of elements"

tools/dom/templates/html/dartium/html_dartium.darttemplate

 // Copyright (c) 2012, 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.
 
 // DO NOT EDIT
 // Auto-generated dart:html library.
 
 /**
  * HTML elements and other resources for web-based applications that need to
  * interact with the browser and the DOM (Document Object Model).
@@ skipping 60 matching lines @@
 part '$AUXILIARY_DIR/KeyCode.dart';
 part '$AUXILIARY_DIR/KeyLocation.dart';
 part '$AUXILIARY_DIR/KeyName.dart';
 part '$AUXILIARY_DIR/KeyboardEventStream.dart';
 part '$AUXILIARY_DIR/NodeValidatorBuilder.dart';
 part '$AUXILIARY_DIR/ReadyState.dart';
 part '$AUXILIARY_DIR/Validators.dart';
 part '$AUXILIARY_DIR/WrappedList.dart';
 part '$AUXILIARY_DIR/_HttpRequestUtils.dart';
 part '$AUXILIARY_DIR/_ListIterators.dart';
-part '$AUXILIARY_DIR/dartium_CustomElementSupport.dart';
 part '$AUXILIARY_DIR/dartium_KeyEvent.dart';
 part '$AUXILIARY_DIR/dartium_Platform.dart';
 part '$AUXILIARY_DIR/shared_html.dart';
 
 part '$AUXILIARY_DIR/native_DOMImplementation.dart';
 
 Window _window;
 
 /**
  * Top-level container for a web page, which is usually a browser tab or window.
@@ skipping 32 matching lines @@
 /**
  * Spawn a DOM isolate using the given URI in the same window.
  * This isolate is not concurrent.  It runs on the browser thread
  * with full access to the DOM.
  * Note: this API is still evolving and may move to dart:isolate.
  */
 @Experimental()
 Future<Isolate> spawnDomUri(Uri uri, List<String> args, message) {
   // TODO(17738): Plumb arguments and return value through.
   return _Utils.spawnDomUri(uri.toString());
-}
+}