Add dummy dart:html library for documentation.

lib/html/doc/interface/NodeSelector.dartdoc

+// Copyright (c) 2011, 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:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName NodeSelector
+interface NodeSelector {
+
+  // TODO(nweiz): add this back once DocumentFragment is ported.
+  // ElementList queryAll(String selectors);
+
+
+  /** @domName NodeSelector.querySelector */
+  Element querySelector(String selectors);
+
+  /** @domName NodeSelector.querySelectorAll */
+  NodeList querySelectorAll(String selectors);
+
+}