Fix all strong-mode warnings.

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.
 ///
 /// When this is cancelled, that means it won't complete either successfully or
 /// with an error, regardless of whether the wrapped Future completes.
 /// Cancelling this won't stop whatever code is feeding the wrapped future from
 /// running.
 class CancelableFuture<T> implements Future<T> {
   bool _canceled = false;
   final _completer = new Completer<T>.sync();
 
   CancelableFuture(Future<T> inner) {
     inner.then((result) {
       if (_canceled) return;
       _completer.complete(result);
     }).catchError((error, stackTrace) {
       if (_canceled) return;
       _completer.completeError(error, stackTrace);
     });
   }
 
   Stream<T> asStream() => _completer.future.asStream();
-  Future catchError(Function onError, {bool test(error)}) =>
+  Future catchError(Function onError, {bool test(Object error)}) =>
     _completer.future.catchError(onError, test: test);
-  Future then(onValue(T value), {Function onError}) =>
+  Future/*<S>*/ then/*<S>*/(/*=S*/ onValue(T value), {Function onError}) =>
     _completer.future.then(onValue, onError: onError);
   Future<T> whenComplete(action()) => _completer.future.whenComplete(action);
-  Future timeout(Duration timeLimit, {void onTimeout()}) =>
+  Future<T> timeout(Duration timeLimit, {void onTimeout()}) =>
     _completer.future.timeout(timeLimit, onTimeout: onTimeout);
   /// Cancels this future.
   void cancel() {
     _canceled = true;
   }
 }