Remove unnecessary dart:async imports (#50)

Since Dart 2.1, Future and Stream have been exported from dart:core
diff --git a/lib/src/interface/local_process_manager.dart b/lib/src/interface/local_process_manager.dart
index 1c8b999..d5133bb 100644
--- a/lib/src/interface/local_process_manager.dart
+++ b/lib/src/interface/local_process_manager.dart
@@ -2,7 +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.
 
-import 'dart:async';
 import 'dart:convert';
 import 'dart:io'
     show
diff --git a/lib/src/interface/process_manager.dart b/lib/src/interface/process_manager.dart
index 67fd079..e218c83 100644
--- a/lib/src/interface/process_manager.dart
+++ b/lib/src/interface/process_manager.dart
@@ -2,7 +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.
 
-import 'dart:async';
 import 'dart:convert';
 import 'dart:io'
     show