Add dummy dart:html library for documentation.

lib/html/doc/interface/NamedNodeMap.dartdoc

+// 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:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName NamedNodeMap
+interface NamedNodeMap extends List<Node> {
+
+  /** @domName NamedNodeMap.length */
+  final int length;
+
+  /** @domName NamedNodeMap.getNamedItem */
+  Node getNamedItem(String name);
+
+  /** @domName NamedNodeMap.getNamedItemNS */
+  Node getNamedItemNS(String namespaceURI, String localName);
+
+  /** @domName NamedNodeMap.item */
+  Node item(int index);
+
+  /** @domName NamedNodeMap.removeNamedItem */
+  Node removeNamedItem(String name);
+
+  /** @domName NamedNodeMap.removeNamedItemNS */
+  Node removeNamedItemNS(String namespaceURI, String localName);
+
+  /** @domName NamedNodeMap.setNamedItem */
+  Node setNamedItem(Node node);
+
+  /** @domName NamedNodeMap.setNamedItemNS */
+  Node setNamedItemNS(Node node);
+}