Add dummy dart:html library for documentation.

lib/html/doc/interface/UIEvent.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 UIEvent
+interface UIEvent extends Event {
+
+  /** @domName UIEvent.charCode */
+  final int charCode;
+
+  /** @domName UIEvent.detail */
+  final int detail;
+
+  /** @domName UIEvent.keyCode */
+  final int keyCode;
+
+  /** @domName UIEvent.layerX */
+  final int layerX;
+
+  /** @domName UIEvent.layerY */
+  final int layerY;
+
+  /** @domName UIEvent.pageX */
+  final int pageX;
+
+  /** @domName UIEvent.pageY */
+  final int pageY;
+
+  /** @domName UIEvent.view */
+  final Window view;
+
+  /** @domName UIEvent.which */
+  final int which;
+
+  /** @domName UIEvent.initUIEvent */
+  void initUIEvent(String type, bool canBubble, bool cancelable, Window view, int detail);
+}