Remove unused dart:async import. (#134)
Since this version doesn't support Dart 2.0, this import is not
diff --git a/lib/src/result/future.dart b/lib/src/result/future.dart
index 20a5ebf..a8dd316 100644
@@ -2,8 +2,6 @@
// 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.