Add a WebSocketChannel class.

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].
+class CompatibleWebSocketException implements Exception {
+  final String message;
+
+  CompatibleWebSocketException([this.message]);
+
+  String toString() => message == null
+      ? "CompatibleWebSocketException" :
+        "CompatibleWebSocketException: $message";
+}