Add dummy dart:html library for documentation.

lib/html/doc/interface/WorkerNavigator.dart

+// 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 WorkerNavigator
+interface WorkerNavigator {
+
+  /** @domName WorkerNavigator.appName */
+  final String appName;
+
+  /** @domName WorkerNavigator.appVersion */
+  final String appVersion;
+
+  /** @domName WorkerNavigator.onLine */
+  final bool onLine;
+
+  /** @domName WorkerNavigator.platform */
+  final String platform;
+
+  /** @domName WorkerNavigator.userAgent */
+  final String userAgent;
+}