Fix a broken test.

test/mock_client_test.dart

Index: test/mock_client_test.dart
diff --git a/test/mock_client_test.dart b/test/mock_client_test.dart
index 84e9af179166c3a3cb14c40286a989ad755c3f1f..a19adcad2485a255498bb66eea27495ab145dfde 100644
--- a/test/mock_client_test.dart
+++ b/test/mock_client_test.dart
@@ -33,7 +33,7 @@ void main() {
     var client = new MockClient.streaming((request, bodyStream) {
       return bodyStream.bytesToString().then((bodyString) {
         var controller = new StreamController<List<int>>(sync: true);
-        async.then((_) {
+        new Future.sync(() {
           controller.add('Request body was "$bodyString"'.codeUnits);
           controller.close();
         });