Add support for custom elements to MDV

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 e4faf0ceb80daa8267c5bc0bd2213b8198ae2e23..8c342fd998e1e3be471d371eaee98110815c8560 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -424,20 +424,29 @@ $(ANNOTATIONS)abstract class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   void onCreated() {}
 
   // Hooks to support custom WebComponents.
+
+$if DART2JS
+  @Creates('Null')  // Set from Dart code; does not instantiate a native type.
+$endif
+  Element _xtag;
+
   /**
    * 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.
+   * 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/
    */
-$if DART2JS
-  @Creates('Null')  // Set from Dart code; does not instantiate a native type.
-$endif
-  var xtag;
+  Element get xtag => _xtag != null ? _xtag : this;
+
+  void set xtag(Element value) {
+    _xtag = value;
+  }
 
   /**
    * Scrolls this element into view.
@@ -674,7 +683,7 @@ $endif
       self._attributeBindings = new Map<String, StreamSubscription>();
     }
 
-    self.attributes.remove(name);
+    self.xtag.attributes.remove(name);
 
     var changed;
     if (name.endsWith('?')) {
@@ -682,16 +691,16 @@ $endif
 
       changed = (value) {
         if (_templateBooleanConversion(value)) {
-          self.attributes[name] = '';
+          self.xtag.attributes[name] = '';
         } else {
-          self.attributes.remove(name);
+          self.xtag.attributes.remove(name);
         }
       };
     } else {
       changed = (value) {
         // TODO(jmesserly): escape value if needed to protect against XSS.
         // See https://github.com/toolkitchen/mdv/issues/58
-        self.attributes[name] = value == null ? '' : '$value';
+        self.xtag.attributes[name] = value == null ? '' : '$value';
       };
     }