Deprecate WebSocketChannel.

lib/src/web_socket/exception.dart

 // Copyright (c) 2016, 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.
 
-import 'channel.dart';
-
-/// An exception thrown by [WebSocketChannel].
+/// This class is deprecated.
+///
+/// Use the [`web_socket_channel`][web_socket_channel] package instead.
+///
+/// [web_socket_channel]: https://pub.dartlang.org/packages/web_socket_channel
+@Deprecated("Will be removed in 3.0.0.")
 class CompatibleWebSocketException implements Exception {
   final String message;
 
   CompatibleWebSocketException([this.message]);
 
   String toString() => message == null
       ? "CompatibleWebSocketException" :
         "CompatibleWebSocketException: $message";
 }