Make new StreamController be async by default.

pkg/scheduled_test/lib/src/utils.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library utils;
 
 import 'dart:async';
 
 import 'package:stack_trace/stack_trace.dart';
 
@@ skipping 63 matching lines @@
 }
 
 // TODO(nweiz): remove this when issue 8731 is fixed.
 /// Returns a [Stream] that will immediately emit [error] and then close.
 Stream errorStream(error) => new Future.error(error).asStream();
 
 /// Returns a buffered stream that will emit the same values as the stream
 /// returned by [future] once [future] completes. If [future] completes to an
 /// error, the return value will emit that error and then close.
 Stream futureStream(Future<Stream> future) {
-  var controller = new StreamController();
+  var controller = new StreamController(sync: true);
   future.then((stream) {
     stream.listen(
         controller.add,
         onError: (error) => controller.addError(error),
         onDone: controller.close);
   }).catchError((e) {
     controller.addError(e);
     controller.close();
   });
   return controller.stream;
@@ skipping 35 matching lines @@
 }
 
 /// A function that can be called to cancel a [Stream] and send a done message.
 typedef void StreamCanceller();
 
 // TODO(nweiz): use a StreamSubscription when issue 9026 is fixed.
 /// Returns a wrapped version of [stream] along with a function that will cancel
 /// the wrapped stream. Unlike [StreamSubscription], this canceller will send a
 /// "done" message to the wrapped stream.
 Pair<Stream, StreamCanceller> streamWithCanceller(Stream stream) {
-  var controller = new StreamController();
+  var controller = new StreamController(sync: true);
   var controllerStream = stream.isBroadcast ?
       controller.stream.asBroadcastStream() :
       controller.stream;
   var subscription = stream.listen((value) {
     if (!controller.isClosed) controller.add(value);
   }, onError: (error) {
     if (!controller.isClosed) controller.addError(error);
   }, onDone: controller.close);
   return new Pair<Stream, StreamCanceller>(controllerStream, controller.close);
 }
 
 // TODO(nweiz): remove this when issue 7787 is fixed.
 /// Creates two single-subscription [Stream]s that each emit all values and
 /// errors from [stream]. This is useful if [stream] is single-subscription but
 /// multiple subscribers are necessary.
 Pair<Stream, Stream> tee(Stream stream) {
-  var controller1 = new StreamController();
-  var controller2 = new StreamController();
+  var controller1 = new StreamController(sync: true);
+  var controller2 = new StreamController(sync: true);
   stream.listen((value) {
     controller1.add(value);
     controller2.add(value);
   }, onError: (error) {
     controller1.addError(error);
     controller2.addError(error);
   }, onDone: () {
     controller1.close();
     controller2.close();
   });
@@ skipping 33 matching lines @@
 }
 
 /// Returns a string representation of [trace] that has the core and test frames
 /// folded together.
 String terseTraceString(StackTrace trace) {
   return new Trace.from(trace).terse.foldFrames((frame) {
     return frame.package == 'scheduled_test' || frame.package == 'unittest' ||
         frame.isCore;
   }).toString().trim();
 }