Roll to pickup pool changes

packages/barback/lib/src/utils/cancelable_future.dart

 // Copyright (c) 2013, 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.
 
 library barback.utils.cancelable_future;
 
 import 'dart:async';
 
 /// A wrapper for [Future] that can be cancelled.
 ///
@@ skipping 11 matching lines @@
       _completer.complete(result);
     }).catchError((error, stackTrace) {
       if (_canceled) return;
       _completer.completeError(error, stackTrace);
     });
   }
 
   Stream<T> asStream() => _completer.future.asStream();
   Future<T> catchError(Function onError, {bool test(Object error)}) =>
       _completer.future.catchError(onError, test: test);
-  Future/*<S>*/ then/*<S>*/(/*=FutureOr<S>*/ onValue(T value),
-          {Function onError}) =>
+  Future<S> then<S>(FutureOr<S> onValue(T value), {Function onError}) =>
       _completer.future.then(onValue, onError: onError);
   Future<T> whenComplete(action()) => _completer.future.whenComplete(action);
   Future<T> timeout(Duration timeLimit, {void onTimeout()}) =>
       _completer.future.timeout(timeLimit, onTimeout: onTimeout);
 
   /// Cancels this future.
   void cancel() {
     _canceled = true;
   }
 }