Add typed wrapper functions to delegate classes.

lib/src/delegate/event_sink.dart

Index: lib/src/delegate/event_sink.dart
diff --git a/lib/src/delegate/event_sink.dart b/lib/src/delegate/event_sink.dart
index 5a525dfc8c55d079a21ef7f4e0c0366134a9889a..14501b28d3ea65906c4cf3ebd51cf5a5c61e74ac 100644
--- a/lib/src/delegate/event_sink.dart
+++ b/lib/src/delegate/event_sink.dart
@@ -12,7 +12,18 @@ class DelegatingEventSink<T> implements EventSink<T> {
   final EventSink _sink;
 
   /// Create a delegating sink forwarding calls to [sink].
-  DelegatingEventSink(EventSink sink) : _sink = sink;
+  DelegatingEventSink(EventSink<T> sink) : _sink = sink;
+
+  DelegatingEventSink._(this._sink);
+
+  /// Creates a wrapper that coerces the type of [sink].
+  ///
+  /// Unlike [new DelegatingEventSink], this only requires its argument to be an
+  /// instance of `EventSink`, not `EventSink<T>`. This means that calls to
+  /// [add] may throw a [CastError] if the argument type doesn't match the
+  /// reified type of [sink].
+  static EventSink/*<T>*/ typed/*<T>*/(EventSink sink) =>
+      sink is EventSink/*<T>*/ ? sink : new DelegatingEventSink._(sink);
 
   void add(T data) {
     _sink.add(data);