Fix Dart 1.17 strong-mode warnings.

lib/src/delegate/stream_subscription.dart

 // Copyright (c) 2015, 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';
 
 import '../typed/stream_subscription.dart';
 
 /// Simple delegating wrapper around a [StreamSubscription].
 ///
@@ skipping 33 matching lines @@
   void pause([Future resumeFuture]) {
     _source.pause(resumeFuture);
   }
 
   void resume() {
     _source.resume();
   }
 
   Future cancel() => _source.cancel();
 
-  Future asFuture([futureValue]) => _source.asFuture(futureValue);
+  Future/*<E>*/ asFuture/*<E>*/([/*=E*/ futureValue]) =>
+      _source.asFuture(futureValue);
 
   bool get isPaused => _source.isPaused;
 }