Fix cancelable_future again.

pkg/barback/lib/src/cancelable_future.dart

-import 'dart:async';
+// 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.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>();
 
@@ skipping 12 matching lines @@
     _completer.future.catchError(onError, test: test);
   Future then(onValue(T value), {onError(error)}) =>
     _completer.future.then(onValue, onError: onError);
   Future<T> whenComplete(action()) => _completer.future.whenComplete(action);
 
   /// Cancels this future.
   void cancel() {
     _canceled = true;
   }
 }