Add support for stream- and sink-specific changes.

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';
 
+import 'package:async/async.dart';
+
 import 'src/stream_channel_transformer.dart';
 
 export 'src/delegating_stream_channel.dart';
 export 'src/isolate_channel.dart';
 export 'src/json_document_transformer.dart';
 export 'src/multi_channel.dart';
 export 'src/stream_channel_completer.dart';
 export 'src/stream_channel_transformer.dart';
 
 /// An abstract class representing a two-way communication channel.
@@ skipping 51 matching lines @@
       new _StreamChannel<T>(stream, sink);
 
   /// Connects [this] to [other], so that any values emitted by either are sent
   /// directly to the other.
   void pipe(StreamChannel<T> other);
 
   /// Transforms [this] using [transformer].
   ///
   /// This is identical to calling `transformer.bind(channel)`.
   StreamChannel transform(StreamChannelTransformer<dynamic, T> transformer);
+
+  /// Transforms only the [stream] component of [this] using [transformer].
+  StreamChannel<T> transformStream(StreamTransformer<T, T> transformer);
+
+  /// Transforms only the [sink] component of [this] using [transformer].
+  StreamChannel<T> transformSink(StreamSinkTransformer<T, T> transformer);
+
+  /// Returns a copy of [this] with [stream] replaced by [change]'s return
+  /// value.
+  StreamChannel<T> changeStream(Stream<T> change(Stream<T> stream));
+
+  /// Returns a copy of [this] with [sink] replaced by [change]'s return
+  /// value.
+  StreamChannel<T> changeSink(StreamSink<T> change(StreamSink<T> sink));
 }
 
 /// An implementation of [StreamChannel] that simply takes a stream and a sink
 /// as parameters.
 ///
 /// This is distinct from [StreamChannel] so that it can use
 /// [StreamChannelMixin].
 class _StreamChannel<T> extends StreamChannelMixin<T> {
   final Stream<T> stream;
   final StreamSink<T> sink;
 
   _StreamChannel(this.stream, this.sink);
 }
 
 /// 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);
   }
 
   StreamChannel transform(StreamChannelTransformer<dynamic, T> transformer) =>
       transformer.bind(this);
+
+  StreamChannel<T> transformStream(StreamTransformer<T, T> transformer) =>
+      changeStream(transformer.bind);
+
+  StreamChannel<T> transformSink(StreamSinkTransformer<T, T> transformer) =>
+      changeSink(transformer.bind);
+
+  StreamChannel<T> changeStream(Stream<T> change(Stream<T> stream)) =>
+      new StreamChannel(change(stream), sink);
+
+  StreamChannel<T> changeSink(StreamSink<T> change(StreamSink<T> sink)) =>
+      new StreamChannel(stream, change(sink));
 }