fix warnings caused by xtag

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 50f53ec0530337de9e22aed055f6f790602f3c3d..69de5cb39ad6334ca8f7e02ef3cf2696bdf6ca5c 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -7402,7 +7402,9 @@ abstract class Element extends Node implements ElementTraversal native "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;