Make new StreamController be async by default.

tests/lib/async/slow_consumer2_test.dart

 // Copyright (c) 2012, 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.
 
 // VMOptions=--old_gen_heap_size=64
 
 library slow_consumer2_test;
 
 import "package:expect/expect.dart";
 import 'dart:async';
@@ skipping 52 matching lines @@
 }
 
 class DataProvider {
   final int chunkSize;
   final int bytesPerSecond;
   int sentCount = 0;
   int targetCount;
   StreamController controller;
 
   DataProvider(int this.bytesPerSecond, int this.targetCount, this.chunkSize) {
-    controller = new StreamController(
+    controller = new StreamController(sync: true,
       onPause: onPauseStateChange,
       onResume: onPauseStateChange);
     Timer.run(send);
   }
 
   Stream get stream => controller.stream;
 
   send() {
     if (controller.isPaused) return;
     if (sentCount == targetCount) {
@@ skipping 30 matching lines @@
   // file). If the consumer doesn't pause the data-provider it will run out of
   // heap-space.
 
   new DataProvider(800 * MB, 100 * MB, 1 * MB).stream
     .pipe(new SlowConsumer(200 * MB, 5 * MB))
     .then((count) {
       port.close();
       Expect.equals(100 * MB, count);
     });
 }