Fix cancelable_future again.

pkg/barback/lib/src/cancelable_future.dart

Index: pkg/barback/lib/src/cancelable_future.dart
diff --git a/pkg/barback/lib/src/cancelable_future.dart b/pkg/barback/lib/src/cancelable_future.dart
index 120c7bd431ac439608d9fec33fd6f23d0ded55d3..043ad6cd8487673ca7caccef7016f92948398b23 100644
--- a/pkg/barback/lib/src/cancelable_future.dart
+++ b/pkg/barback/lib/src/cancelable_future.dart
@@ -1,7 +1,11 @@
-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