Make new StreamController be async by default.

pkg/http/test/response_test.dart

Index: pkg/http/test/response_test.dart
diff --git a/pkg/http/test/response_test.dart b/pkg/http/test/response_test.dart
index 17b2b60534665f41ffeefaa3c9c9f214ae899f1a..5fafa37a3dd4c69fa035002a7e73e9d4915868d2 100644
--- a/pkg/http/test/response_test.dart
+++ b/pkg/http/test/response_test.dart
@@ -44,7 +44,7 @@ void main() {
 
   group('.fromStream()', () {
     test('sets body', () {
-      var controller = new StreamController();
+      var controller = new StreamController(sync: true);
       var streamResponse = new http.StreamedResponse(
           controller.stream, 200, 13);
       var future = http.Response.fromStream(streamResponse)
@@ -57,7 +57,7 @@ void main() {
     });
 
     test('sets bodyBytes', () {
-      var controller = new StreamController();
+      var controller = new StreamController(sync: true);
       var streamResponse = new http.StreamedResponse(controller.stream, 200, 5);
       var future = http.Response.fromStream(streamResponse)
         .then((response) => response.bodyBytes);