Add type-coercion functions.

lib/src/transformer/typed.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 '../../stream_channel.dart';
+
+/// A wrapper that coerces the generic type of the channel returned by an inner
+/// transformer to `S`.
+class TypeSafeStreamChannelTransformer<S, T>
+    implements StreamChannelTransformer<S, T> {
+ final StreamChannelTransformer _inner;
+
+  TypeSafeStreamChannelTransformer(this._inner);
+
+  StreamChannel<S> bind(StreamChannel<T> channel) =>
+      _inner.bind(channel).cast();
+}