Add dummy dart:html library for documentation.

lib/html/doc/interface/DataTransferItem.dart

Index: lib/html/doc/interface/DataTransferItem.dart
diff --git a/lib/html/doc/interface/DataTransferItem.dart b/lib/html/doc/interface/DataTransferItem.dart
new file mode 100644
index 0000000000000000000000000000000000000000..01cd4e2f6613396f019e55e7b2131ac47b84c31f
--- /dev/null
+++ b/lib/html/doc/interface/DataTransferItem.dart
@@ -0,0 +1,21 @@
+// 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: Do not edit - generated code.
+
+/// @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]);
+}