Add a DelegatingStreamChannel class.

lib/stream_channel.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 'dart:async';
 
+export 'src/delegating_stream_channel.dart';
 export 'src/multi_channel.dart';
 
 /// An abstract class representing a two-way communication channel.
 ///
 /// Subclasses are strongly encouraged to mix in or extend [StreamChannelMixin]
 /// to get default implementations of the various instance methods. Adding new
 /// methods to this interface will not be considered a breaking change if
 /// implementations are also added to [StreamChannelMixin].
 abstract class StreamChannel<T> {
   /// The stream that emits values from the other endpoint.
@@ skipping 24 matching lines @@
 }
 
 /// A mixin that implements the instance methods of [StreamChannel] in terms of
 /// [stream] and [sink].
 abstract class StreamChannelMixin<T> implements StreamChannel<T> {
   void pipe(StreamChannel<T> other) {
     stream.pipe(other.sink);
     other.stream.pipe(sink);
   }
 }