Add StreamChannel.withCloseGuarantee.

lib/src/stream_channel_transformer.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 'dart:convert';
 
 import 'package:async/async.dart';
 
 import '../stream_channel.dart';
 import 'transformer/typed.dart';
 
 /// A [StreamChannelTransformer] transforms the events being passed to and
 /// emitted by a [StreamChannel].
 ///
 /// This works on the same principle as [StreamTransformer] and
 /// [StreamSinkTransformer]. Each transformer defines a [bind] method that takes
 /// in the original [StreamChannel] and returns the transformed version.
 ///
-/// Transformers must be able to have `bind` called multiple times.
+/// Transformers must be able to have [bind] called multiple times. If a
+/// subclass implements [bind] explicitly, it should be sure that the returned
+/// stream follows the second stream channel guarantee: closing the sink causes
+/// the stream to close before it emits any more events. This guarantee is
+/// invalidated when an asynchronous gap is added between the original stream's
+/// event dispatch and the returned stream's, for example by transforming it
+/// with a [StreamTransformer]. The guarantee can be easily preserved using [new
+/// StreamChannel.withCloseGuarantee].
 class StreamChannelTransformer<S, T> {
   /// The transformer to use on the channel's stream.
   final StreamTransformer<T, S> _streamTransformer;
 
   /// The transformer to use on the channel's sink.
   final StreamSinkTransformer<S, T> _sinkTransformer;
 
   /// Creates a wrapper that coerces the type of [transformer].
   ///
   /// This soundly converts a [StreamChannelTransformer] to a
@@ skipping 25 matching lines @@
               new StreamSinkTransformer.fromStreamTransformer(codec.encoder)));
 
   /// Transforms the events sent to and emitted by [channel].
   ///
   /// Creates a new channel. When events are passed to the returned channel's
   /// sink, the transformer will transform them and pass the transformed
   /// versions to `channel.sink`. When events are emitted from the
   /// `channel.straem`, the transformer will transform them and pass the
   /// transformed versions to the returned channel's stream.
   StreamChannel<S> bind(StreamChannel<T> channel) =>
-      new StreamChannel<S>(
+      new StreamChannel<S>.withCloseGuarantee(
           channel.stream.transform(_streamTransformer),
           _sinkTransformer.bind(channel.sink));
 }