Make new StreamController be async by default.

tests/standalone/io/stdout_stderr_test.dart

Index: tests/standalone/io/stdout_stderr_test.dart
diff --git a/tests/standalone/io/stdout_stderr_test.dart b/tests/standalone/io/stdout_stderr_test.dart
index ccd1550435184701174bd57c829220f6dd7eb6b2..1f7ac91bf27613251e90b7356a61cea1ceeb5053 100644
--- a/tests/standalone/io/stdout_stderr_test.dart
+++ b/tests/standalone/io/stdout_stderr_test.dart
@@ -16,14 +16,14 @@ callIOSink(IOSink sink) {
   sink.writeCharCode(72);
   sink.add([101, 108, 108, 111, 10]);
 
-  var controller = new StreamController();
+  var controller = new StreamController(sync: true);
   var future = sink.addStream(controller.stream);
   controller.add([72, 101, 108]);
   controller.add([108, 111, 10]);
   controller.close();
 
   future.then((_) {
-    controller = new StreamController();
+    controller = new StreamController(sync: true);
     controller.stream.pipe(sink);
     controller.add([72, 101, 108]);
     controller.add([108, 111, 10]);