Add dummy dart:html library for documentation.

lib/html/doc/interface/DataTransferItem.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 DataTransferItem
+interface DataTransferItem {
+
+  /** @domName DataTransferItem.kind */
+  final String kind;
+
+  /** @domName DataTransferItem.type */
+  final String type;
+
+  /** @domName DataTransferItem.getAsFile */
+  Blob getAsFile();
+
+  /** @domName DataTransferItem.getAsString */
+  void getAsString([StringCallback callback]);
+}