Add type-coercion functions.

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/guarantee_channel.dart';
 import 'src/stream_channel_transformer.dart';
@@ skipping 91 matching lines @@
   /// 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));
+
+  /// Returns a copy of [this] with the generic type coerced to [S].
+  ///
+  /// If any events emitted by [stream] aren't of type [S], they're converted
+  /// into [CastError] events. Similarly, if any events are added to [sync] that
+  /// aren't of type [S], a [CastError] is thrown.
+  StreamChannel/*<S>*/ cast/*<S>*/();
 }
 
 /// 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;
@@ skipping 16 matching lines @@
       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));
+
+  StreamChannel/*<S>*/ cast/*<S>*/() => new StreamChannel(
+      DelegatingStream.typed(stream), DelegatingStreamSink.typed(sink));
 }