Make new StreamController be async by default.

pkg/http/test/mock_client_test.dart

Index: pkg/http/test/mock_client_test.dart
diff --git a/pkg/http/test/mock_client_test.dart b/pkg/http/test/mock_client_test.dart
index 8e46ccc00e90987e89fcc2393b8ff4b1a8226c68..f9f7769b9ca091b8ecd9ba6537d96e237cbcd2d3 100644
--- a/pkg/http/test/mock_client_test.dart
+++ b/pkg/http/test/mock_client_test.dart
@@ -35,7 +35,7 @@ void main() {
   test('handles a streamed request', () {
     var client = new MockClient.streaming((request, bodyStream) {
       return bodyStream.bytesToString().then((bodyString) {
-        var controller = new StreamController<List<int>>();
+        var controller = new StreamController<List<int>>(sync: true);
         async.then((_) {
           controller.add('Request body was "$bodyString"'.codeUnits);
           controller.close();