Add dummy dart:html library for documentation.

lib/html/doc/interface/Clipboard.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 Clipboard
+interface Clipboard {
+
+  /** @domName Clipboard.dropEffect */
+  String dropEffect;
+
+  /** @domName Clipboard.effectAllowed */
+  String effectAllowed;
+
+  /** @domName Clipboard.files */
+  final FileList files;
+
+  /** @domName Clipboard.items */
+  final DataTransferItemList items;
+
+  /** @domName Clipboard.types */
+  final List types;
+
+  /** @domName Clipboard.clearData */
+  void clearData([String type]);
+
+  /** @domName Clipboard.getData */
+  String getData(String type);
+
+  /** @domName Clipboard.setData */
+  bool setData(String type, String data);
+
+  /** @domName Clipboard.setDragImage */
+  void setDragImage(ImageElement image, int x, int y);
+}