Fix Dart 1.17 strong-mode warnings.

lib/src/typed/stream_subscription.dart

Index: lib/src/typed/stream_subscription.dart
diff --git a/lib/src/typed/stream_subscription.dart b/lib/src/typed/stream_subscription.dart
index 800a4badd48f114ccf8c8aa9a4454ba5c2487082..2cd4ddfd1cf57ffc19bb458f3d1168fc40f3487c 100644
--- a/lib/src/typed/stream_subscription.dart
+++ b/lib/src/typed/stream_subscription.dart
@@ -32,5 +32,7 @@ class TypeSafeStreamSubscription<T> implements StreamSubscription<T> {
   }
 
   Future cancel() => _subscription.cancel();
-  Future asFuture([futureValue]) => _subscription.asFuture(futureValue);
+
+  Future/*<E>*/ asFuture/*<E>*/([/*=E*/ futureValue]) =>
+      _subscription.asFuture(futureValue);
 }