Add a DelegatingStreamChannel class.

lib/src/delegating_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.
+
+/// A simple delegating wrapper around [StreamChannel].
+///
+/// Subclasses can override individual methods, or use this to expose only
+/// [StreamChannel] methods.
+class StreamChannelView<T> extends StreamChannelMixin<T> {
+  /// The inner channel to which methods are forwarded.
+  final StreamChannel<T> _inner;
+
+  Stream<T> get stream => _inner.stream;
+  StreamSink<T> get sink => _inner.sink;
+
+  StreamChannelView(this._inner);
+}