fix warnings caused by xtag

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 424 matching lines @@
    * reference to the component implementation. It was inspired by Mozilla's
    * [x-tags][] project. Please note: in the future it may be possible to
    * `extend Element` from your class, in which case this field will be
    * deprecated.
    *
    * If xtag has not been set, it will simply return `this` [Element].
    *
    * [wc]: http://dvcs.w3.org/hg/webcomponents/raw-file/tip/explainer/index.html
    * [x-tags]: http://x-tags.org/
    */
-  Element get xtag => _xtag != null ? _xtag : this;
+  // Note: return type is `dynamic` for convenience to suppress warnings when
+  // members of the component are used. The actual type is a subtype of Element.
+  get xtag => _xtag != null ? _xtag : this;
 
   void set xtag(Element value) {
     _xtag = value;
   }
 
   /**
    * Scrolls this element into view.
    *
    * Only one of of the alignment options may be specified at a time.
    *
@@ skipping 576 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');
 }