Add typed wrapper functions to delegate classes.

lib/src/delegate/stream_subscription.dart

Index: lib/src/delegate/stream_subscription.dart
diff --git a/lib/src/delegate/stream_subscription.dart b/lib/src/delegate/stream_subscription.dart
index e7153b2a6db6a640547abec2e5879f592cfbfba0..b86e91f9192bf9bdc3a3a38f79bbba581b4e83fd 100644
--- a/lib/src/delegate/stream_subscription.dart
+++ b/lib/src/delegate/stream_subscription.dart
@@ -4,6 +4,8 @@
 
 import 'dart:async';
 
+import '../typed/stream_subscription.dart';
+
 /// Simple delegating wrapper around a [StreamSubscription].
 ///
 /// Subclasses can override individual methods.
@@ -11,9 +13,22 @@ class DelegatingStreamSubscription<T> implements StreamSubscription<T> {
   final StreamSubscription _source;
 
   /// Create delegating subscription forwarding calls to [sourceSubscription].
-  DelegatingStreamSubscription(StreamSubscription sourceSubscription)
+  DelegatingStreamSubscription(StreamSubscription<T> sourceSubscription)
       : _source = sourceSubscription;
 
+  /// Creates a wrapper which throws if [subscription]'s events aren't instances
+  /// of `T`.
+  ///
+  /// This soundly converts a [StreamSubscription] to a `StreamSubscription<T>`,
+  /// regardless of its original generic type, by asserting that its events are
+  /// instances of `T` whenever they're provided. If they're not, the
+  /// subscription throws a [CastError].
+  static StreamSubscription/*<T>*/ typed/*<T>*/(
+          StreamSubscription subscription) =>
+      subscription is StreamSubscription/*<T>*/
+          ? subscription
+          : new TypeSafeStreamSubscription/*<T>*/(subscription);
+
   void onData(void handleData(T data)) {
     _source.onData(handleData);
   }