Move _HTMLElement to HtmlElement

tools/dom/templates/html/dart2js/html_dart2js.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 - unless you are editing documentation as per:
 // https://code.google.com/p/dart/wiki/ContributingHTMLDocumentation
 // Auto-generated dart:html library.
 
 /// The Dart HTML library.
 ///
@@ skipping 76 matching lines @@
 /**
  * Root node for all content in a web page.
  */
 HtmlDocument get document => JS('HtmlDocument', 'document');
 
 Element query(String selector) => document.query(selector);
 ElementList queryAll(String selector) => document.queryAll(selector);
 
 // Workaround for tags like <cite> that lack their own Element subclass --
 // Dart issue 1990.
-class _HTMLElement extends Element native "HTMLElement" {
-  factory _HTMLElement() { throw new UnsupportedError("Not supported"); }
+class HtmlElement extends Element native "HTMLElement" {
+  factory HtmlElement() { throw new UnsupportedError("Not supported"); }
 }
 
 // Support for Send/ReceivePortSync.
 int _getNewIsolateId() {
   if (JS('bool', r'!window.$dart$isolate$counter')) {
     JS('void', r'window.$dart$isolate$counter = 1');
   }
   return JS('int', r'window.$dart$isolate$counter++');
 }
 
 // Fast path to invoke JS send port.
 _callPortSync(int id, message) {
   return JS('var', r'ReceivePortSync.dispatchCall(#, #)', id, message);
 }
 
 Future<SendPort> spawnDomFunction(Function f) =>
   new Future.value(IsolateNatives.spawnDomFunction(f));