Add type-coercion functions.

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.
 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
+  /// `StreamChannelTransformer<S, T>`, regardless of its original generic type,
+  /// by asserting that the events emitted by the transformed channel's stream
+  /// are instances of `T` whenever they're provided. If they're not, the stream
+  /// throws a [CastError]. This also means that calls to [StreamSink.add] on
+  /// the transformed channel's sink may throw a [CastError] if the argument
+  /// type doesn't match the reified type of the sink.
+  static StreamChannelTransformer/*<S, T>*/ typed/*<S, T>*/(
+          StreamChannelTransformer transformer) =>
+      transformer is StreamChannelTransformer/*<S, T>*/
+          ? transformer
+          : new TypeSafeStreamChannelTransformer(transformer);
+
   /// Creates a [StreamChannelTransformer] from existing stream and sink
   /// transformers.
   const StreamChannelTransformer(
       this._streamTransformer, this._sinkTransformer);
 
   /// Creates a [StreamChannelTransformer] from a codec's encoder and decoder.
   ///
   /// All input to the inner channel's sink is encoded using [Codec.encoder],
   /// and all output from its stream is decoded using [Codec.decoder].
   StreamChannelTransformer.fromCodec(Codec<S, T> codec)
       : this(
           typedStreamTransformer(codec.decoder),
           StreamSinkTransformer.typed(
               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>(
           channel.stream.transform(_streamTransformer),
           _sinkTransformer.bind(channel.sink));
 }