Add typed wrapper functions to delegate classes.

lib/src/delegate/stream_sink.dart

 // Copyright (c) 2015, 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';
 
 /// Simple delegating wrapper around a [StreamSink].
 ///
 /// Subclasses can override individual methods, or use this to expose only the
 /// [StreamSink] methods of a subclass.
 class DelegatingStreamSink<T> implements StreamSink<T> {
   final StreamSink _sink;
 
   Future get done => _sink.done;
 
   /// Create delegating sink forwarding calls to [sink].
-  DelegatingStreamSink(StreamSink sink)
-      : _sink = sink;
+  DelegatingStreamSink(StreamSink<T> sink) : _sink = sink;
+
+  DelegatingStreamSink._(this._sink);
+
+  /// Creates a wrapper that coerces the type of [sink].
+  ///
+  /// Unlike [new StreamSink], this only requires its argument to be an instance
+  /// of `StreamSink`, not `StreamSink<T>`. This means that calls to [add] may
+  /// throw a [CastError] if the argument type doesn't match the reified type of
+  /// [sink].
+  static StreamSink/*<T>*/ typed/*<T>*/(StreamSink sink) =>
+      sink is StreamSink/*<T>*/ ? sink : new DelegatingStreamSink._(sink);
 
   void add(T data) {
     _sink.add(data);
   }
 
   void addError(error, [StackTrace stackTrace]) {
     _sink.addError(error, stackTrace);
   }
 
   Future addStream(Stream<T> stream) => _sink.addStream(stream);
 
   Future close() => _sink.close();
 }