Add type-coercion functions.

lib/src/stream_channel_transformer.dart

Index: lib/src/stream_channel_transformer.dart
diff --git a/lib/src/stream_channel_transformer.dart b/lib/src/stream_channel_transformer.dart
index ac980859daa2f48564a2ebc391cedd3537c3ddd8..ca09ea16bec34d6fb00dc0cd29f72848a7bb04ce 100644
--- a/lib/src/stream_channel_transformer.dart
+++ b/lib/src/stream_channel_transformer.dart
@@ -8,6 +8,7 @@ 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].
@@ -24,6 +25,21 @@ class StreamChannelTransformer<S, T> {
   /// 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(