add Element.xtag to public interface

lib/html/templates/html/interface/interface_Element.darttemplate

Index: lib/html/templates/html/interface/interface_Element.darttemplate
diff --git a/lib/html/templates/html/interface/interface_Element.darttemplate b/lib/html/templates/html/interface/interface_Element.darttemplate
index 3cdb7017e3db71a093b98337cf9fe74435917db8..b44ce0a5a5fea05f63f80025a39c35120a2edcab 100644
--- a/lib/html/templates/html/interface/interface_Element.darttemplate
+++ b/lib/html/templates/html/interface/interface_Element.darttemplate
@@ -115,5 +115,17 @@ abstract class Element implements Node, NodeSelector {
 
   Element get parent;
 
+  /**
+   * Experimental support for [web components][wc]. This field stores a
+   * 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 and will simply return this [Element] object.
+   *
+   * [wc]: http://dvcs.w3.org/hg/webcomponents/raw-file/tip/explainer/index.html
+   * [x-tags]: http://x-tags.org/
+   */
+  var xtag;
+
 $!MEMBERS
 }