Provide noSuchMethod hook for UnknownElement

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

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Temporary dispatch hook to support WebComponents.
+Function dynamicUnknownElementDispatcher;
+
+class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
+$!MEMBERS
+
+  // Hooks to support custom WebComponents.
+  var xtag;
+
+  noSuchMethod(String name, List args) {
+    if (dynamicUnknownElementDispatcher == null) {
+      throw new NoSuchMethodException(this, name, args);
+    } else {
+      return dynamicUnknownElementDispatcher(this, name, args);
+    }
+  }
+}