Fix Dart 1.17 strong-mode warnings.

lib/src/typed/stream_subscription.dart

 // Copyright (c) 2016, 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';
 
 class TypeSafeStreamSubscription<T> implements StreamSubscription<T> {
   final StreamSubscription _subscription;
 
   bool get isPaused => _subscription.isPaused;
@@ skipping 14 matching lines @@
 
   void pause([Future resumeFuture]) {
     _subscription.pause(resumeFuture);
   }
 
   void resume() {
     _subscription.resume();
   }
 
   Future cancel() => _subscription.cancel();
-  Future asFuture([futureValue]) => _subscription.asFuture(futureValue);
+
+  Future/*<E>*/ asFuture/*<E>*/([/*=E*/ futureValue]) =>
+      _subscription.asFuture(futureValue);
 }