Add an IO implementation of WebSocketChannel.

lib/src/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 a [WebSocketChannel].
 class WebSocketChannelException implements Exception {
   final String message;
 
   /// The exception that caused this one, if available.
   final inner;
 
-  WebSocketChannelException([this.message, this.inner]);
+  WebSocketChannelException([this.message]) : inner = null;
+
+  WebSocketChannelException.from(inner)
+      : message = inner.toString(),
+        inner = inner;
 
   String toString() => message == null
       ? "WebSocketChannelException" :
         "WebSocketChannelException: $message";
 }