Add dummy dart:html library for documentation.

lib/html/doc/interface/NodeList.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 NodeList
+interface NodeList extends List<Node> {
+
+  NodeList filter(bool f(Node element));
+
+  NodeList getRange(int start, int length);
+
+  Node get first();
+
+
+  /** @domName NodeList.length */
+  final int length;
+
+}