Remove unused dart:async import (#53)

diff --git a/test/utils.dart b/test/utils.dart
index cadbf4e..9685cb1 100644
--- a/test/utils.dart
+++ b/test/utils.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';
 
 /// Decodes a UTF8-encoded byte array into a list of Strings, where each list