fix warnings caused by xtag

tools/dom/templates/html/impl/impl_Element.darttemplate

Index: tools/dom/templates/html/impl/impl_Element.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Element.darttemplate b/tools/dom/templates/html/impl/impl_Element.darttemplate
index 425669b037a0a759acc7fcf3b47a964aec9d94f6..261a6cd5eca48dee29f522081e3b10217664b553 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -442,7 +442,9 @@ $endif
    * [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;