Reorganize barback's source files.

pkg/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.cancelable_future;
+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> {
@@ skipping 16 matching lines @@
   Future then(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()}) =>
     _completer.future.timeout(timeLimit, onTimeout: onTimeout);
   /// Cancels this future.
   void cancel() {
     _canceled = true;
   }
 }