Minor tweaks to dart:html for better dart2js codegen

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.
 
 /**
  * HTML elements and other resources for web-based applications that need to
@@ skipping 98 matching lines @@
  *
  * See also:
  *
  *   * [Window](https://developer.mozilla.org/en-US/docs/Web/API/window) from MDN.
  */
 Window get window => JS('Window', 'window');
 
 /**
  * Root node for all content in a web page.
  */
-HtmlDocument get document => JS('HtmlDocument', 'document');
+HtmlDocument get document =>
+    JS('returns:HtmlDocument;depends:none;effects:none;gvn:true', 'document');
 
 // Workaround for tags like <cite> that lack their own Element subclass --
 // Dart issue 1990.
 @Native("HTMLElement")
 class HtmlElement extends Element {
   factory HtmlElement() { throw new UnsupportedError("Not supported"); }
 
   /**
    * Constructor instantiated by the DOM when a custom element has been created.
    *
    * This can only be called by subclasses from their created constructor.
    */
   HtmlElement.created() : super.created();
 }
 
 /**
  * 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): Implement this.
   throw new UnimplementedError();
 }
 
 createCustomUpgrader(Type customElementClass, $this) => $this;