Fixing Element.shadowRoot JS type annotation

tools/dom/templates/html/impl/impl_Element.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.
 
 part of $LIBRARYNAME;
 
 class _ChildrenElementList extends ListBase<Element> {
   // Raw Element.
   final Element _element;
   final HtmlCollection _childElements;
@@ skipping 926 matching lines @@
   ShadowRoot createShadowRoot() {
     return JS('ShadowRoot',
       '(#.createShadowRoot || #.webkitCreateShadowRoot).call(#)',
       this, this, this);
   }
 
   @DomName('Element.shadowRoot')
   @SupportedBrowser(SupportedBrowser.CHROME, '25')
   @Experimental()
   ShadowRoot get shadowRoot =>
-      JS('ShadowRoot', '#.shadowRoot || #.webkitShadowRoot', this, this);
+      JS('ShadowRoot|Null', '#.shadowRoot || #.webkitShadowRoot', this, this);
 $endif
 
 
   /**
    * Gets the template this node refers to.
    * This is only supported if [isTemplate] is true.
    */
   @Experimental()
   Element get ref {
     _ensureTemplate();
@@ skipping 397 matching lines @@
   const ScrollAlignment._internal(this._value);
   toString() => 'ScrollAlignment.$_value';
 
   /// Attempt to align the element to the top of the scrollable area.
   static const TOP = const ScrollAlignment._internal('TOP');
   /// Attempt to center the element in the scrollable area.
   static const CENTER = const ScrollAlignment._internal('CENTER');
   /// Attempt to align the element to the bottom of the scrollable area.
   static const BOTTOM = const ScrollAlignment._internal('BOTTOM');
 }