Add dummy dart:html library for documentation.

lib/html/doc/interface/DataTransferItemList.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 DataTransferItemList
+interface DataTransferItemList {
+
+  /** @domName DataTransferItemList.length */
+  final int length;
+
+  /** @domName DataTransferItemList.add */
+  void add(data_OR_file, [String type]);
+
+  /** @domName DataTransferItemList.clear */
+  void clear();
+
+  /** @domName DataTransferItemList.item */
+  DataTransferItem item(int index);
+}