Add dummy dart:html library for documentation.

lib/html/doc/interface/Storage.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.
+
+/// @domName Storage
+interface Storage extends Map<String, String> {
+
+  /** @domName Storage.length */
+  final int $dom_length;
+
+  /** @domName Storage.clear */
+  void $dom_clear();
+
+  /** @domName Storage.getItem */
+  String $dom_getItem(String key);
+
+  /** @domName Storage.key */
+  String $dom_key(int index);
+
+  /** @domName Storage.removeItem */
+  void $dom_removeItem(String key);
+
+  /** @domName Storage.setItem */
+  void $dom_setItem(String key, String data);
+
+}