fix warnings caused by xtag

sdk/lib/html/dart2js/html_dart2js.dart

 /// The Dart HTML library.
 library dart.dom.html;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:_collection-dev' hide Symbol;
 import 'dart:html_common';
 import 'dart:indexed_db';
 import 'dart:isolate';
 import 'dart:json' as json;
@@ skipping 7384 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 20581 matching lines @@
       _position = nextPosition;
       return true;
     }
     _current = null;
     _position = _array.length;
     return false;
   }
 
   T get current => _current;
 }