Move noSuchMethod hook from UnknownElement to Element

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

Index: lib/dom/templates/html/impl/impl_Element.darttemplate
diff --git a/lib/dom/templates/html/impl/impl_Element.darttemplate b/lib/dom/templates/html/impl/impl_Element.darttemplate
index 61d282377280d1a561a457c6da7515df696ee224..fc2842d02aec158e2ee99ab902f5c8115768eec6 100644
--- a/lib/dom/templates/html/impl/impl_Element.darttemplate
+++ b/lib/dom/templates/html/impl/impl_Element.darttemplate
@@ -719,9 +719,28 @@ class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
         () => _window.$dom_getComputedStyle(this, pseudoElement),
         new Completer<CSSStyleDeclaration>());
   }
+
+  // Hooks to support custom WebComponents.
+  var xtag;
+
+$if DARTIUM
+  noSuchMethod(String name, List args) {
+    if (dynamicUnknownElementDispatcher == null) {
+      throw new NoSuchMethodException(this, name, args);
+    } else {
+      return dynamicUnknownElementDispatcher(this, name, args);
+    }
+  }
+$else
+  // TODO(vsm): Implement noSuchMethod or similar for dart2js.
+$endif
+
 $!MEMBERS
 }
 
+// Temporary dispatch hook to support WebComponents.
+Function dynamicUnknownElementDispatcher;
+
 final _START_TAG_REGEXP = const RegExp('<(\\w+)');
 class _ElementFactoryProvider {
   static final _CUSTOM_PARENT_TAG_MAP = const {