Remove unused dart:async import

No longer needed since Dart 2.1 added Future/Stream to dart:core exports
diff --git a/test/utils.dart b/test/utils.dart
index 2d1ab06..b22f73f 100644
--- a/test/utils.dart
+++ b/test/utils.dart
@@ -4,7 +4,6 @@
 
 library dart_style.test.utils;
 
-import 'dart:async';
 import 'dart:io';
 import 'dart:mirrors';