Fix a broken test.

R=alanknight@google.com

Review URL: https://codereview.chromium.org//1947793002 .
diff --git a/pubspec.yaml b/pubspec.yaml
index 574cf00..5f39657 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: http
-version: 0.11.3+6
+version: 0.11.4-dev
 author: "Dart Team <misc@dartlang.org>"
 homepage: https://github.com/dart-lang/http
 description: A composable, Future-based API for making HTTP requests.
diff --git a/test/mock_client_test.dart b/test/mock_client_test.dart
index 84e9af1..a19adca 100644
--- a/test/mock_client_test.dart
+++ b/test/mock_client_test.dart
@@ -33,7 +33,7 @@
     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();
         });