Remove unused dart:async import

Since Dart 2.1, Future/Stream have been exported from dart:core

pkgs/glob/lib/glob.dart

diff --git a/pkgs/glob/lib/glob.dart b/pkgs/glob/lib/glob.dart
index 6f6add6..75bedd6 100644
--- a/pkgs/glob/lib/glob.dart
+++ b/pkgs/glob/lib/glob.dart
@@ -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.
 
-import 'dart:async';
-
 import 'package:path/path.dart' as p;
 
 import 'src/ast.dart';