Add APIs for asserting the types of transformers.

lib/src/stream_sink_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 'dart:async';
+
+import '../delegate/stream_sink.dart';
+import '../stream_sink_transformer.dart';
+
+/// A wrapper that coerces the generic type of the sink returned by an inner
+/// transformer to `S`.
+class TypeSafeStreamSinkTransformer<S, T>
+    implements StreamSinkTransformer<S, T> {
+  final StreamSinkTransformer _inner;
+
+  TypeSafeStreamSinkTransformer(this._inner);
+
+  StreamSink<S> bind(StreamSink<T> sink) =>
+      DelegatingStreamSink.typed(_inner.bind(sink));
+}