Add dummy dart:html library for documentation.

lib/html/doc/interface/Touch.dartdoc

Index: lib/html/doc/interface/Touch.dartdoc
diff --git a/lib/html/doc/interface/Touch.dartdoc b/lib/html/doc/interface/Touch.dartdoc
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+// 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 Touch
+interface Touch {
+
+  /** @domName Touch.clientX */
+  final int clientX;
+
+  /** @domName Touch.clientY */
+  final int clientY;
+
+  /** @domName Touch.identifier */
+  final int identifier;
+
+  /** @domName Touch.pageX */
+  final int pageX;
+
+  /** @domName Touch.pageY */
+  final int pageY;
+
+  /** @domName Touch.screenX */
+  final int screenX;
+
+  /** @domName Touch.screenY */
+  final int screenY;
+
+  /** @domName Touch.target */
+  final EventTarget target;
+
+  /** @domName Touch.webkitForce */
+  final num webkitForce;
+
+  /** @domName Touch.webkitRadiusX */
+  final int webkitRadiusX;
+
+  /** @domName Touch.webkitRadiusY */
+  final int webkitRadiusY;
+
+  /** @domName Touch.webkitRotationAngle */
+  final num webkitRotationAngle;
+}