Remove another unused dart:async import. (#412)

diff --git a/protoc_plugin/test/service_test.dart b/protoc_plugin/test/service_test.dart
index 08b23bf..dffe573 100644
--- a/protoc_plugin/test/service_test.dart
+++ b/protoc_plugin/test/service_test.dart
@@ -1,7 +1,5 @@
 library service_test;
 
-import 'dart:async' show Future;
-
 import 'package:protobuf/protobuf.dart';
 import 'package:protoc_plugin/src/descriptor.pb.dart'
     show DescriptorProto, ServiceDescriptorProto;