Tighten type of 'document' to HtmlDocument

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.
 library dart.dom.html;
@@ skipping 47 matching lines @@
 
 
 /**
  * The top-level Window object.
  */
 Window get window => JS('Window', 'window');
 
 /**
  * The top-level Document object.
  */
-HtmlDocument get document => JS('Document', 'document');
+HtmlDocument get document => JS('HtmlDocument', 'document');
 
 Element query(String selector) => document.query(selector);
 List<Element> 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" {
 }
 
 // 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);
 }
 
 spawnDomFunction(f) => IsolateNatives.spawnDomFunction(f);