Add typed wrapper functions to delegate classes.

lib/src/delegate/stream_sink.dart

Index: lib/src/delegate/stream_sink.dart
diff --git a/lib/src/delegate/stream_sink.dart b/lib/src/delegate/stream_sink.dart
index e06afc1b0a611c919a0d7fa6d9b69a61cecd7be7..9b52b19e5d5e642dca21e3e1b2ab812cbd8056df 100644
--- a/lib/src/delegate/stream_sink.dart
+++ b/lib/src/delegate/stream_sink.dart
@@ -14,8 +14,18 @@ class DelegatingStreamSink<T> implements StreamSink<T> {
   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);