Add dummy dart:html library for documentation.

lib/html/doc/interface/ElementTraversal.dart

+// 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.
+
+// WARNING: Do not edit - generated code.
+
+/// @domName ElementTraversal
+interface ElementTraversal {
+
+  /** @domName ElementTraversal.childElementCount */
+  final int childElementCount;
+
+  /** @domName ElementTraversal.firstElementChild */
+  final Element firstElementChild;
+
+  /** @domName ElementTraversal.lastElementChild */
+  final Element lastElementChild;
+
+  /** @domName ElementTraversal.nextElementSibling */
+  final Element nextElementSibling;
+
+  /** @domName ElementTraversal.previousElementSibling */
+  final Element previousElementSibling;
+}