Make new StreamController be async by default.

sdk/lib/_internal/pub/lib/src/utils.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.
 
 /// Generic utility functions. Stuff that should possibly be in core.
 library utils;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
@@ skipping 165 matching lines @@
     completer.completeError(e);
   }, onDone: () {
     completer.completeError(new StateError("No elements"));
   }, cancelOnError: true);
   return completer.future;
 }
 
 /// Returns a wrapped version of [stream] along with a [StreamSubscription] that
 /// can be used to control the wrapped stream.
 Pair<Stream, StreamSubscription> streamWithSubscription(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(controller.add,
       onError: controller.addError,
       onDone: controller.close);
   return new Pair<Stream, StreamSubscription>(controllerStream, subscription);
 }
 
 // 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();
   });
   return new Pair<Stream, Stream>(controller1.stream, controller2.stream);
 }
 
 /// Merges [stream1] and [stream2] into a single stream that emits events from
 /// both sources.
 Stream mergeStreams(Stream stream1, Stream stream2) {
   var doneCount = 0;
-  var controller = new StreamController();
+  var controller = new StreamController(sync: true);
 
   for (var stream in [stream1, stream2]) {
     stream.listen((value) {
       controller.add(value);
     }, onError: (error) {
       controller.addError(error);
     }, onDone: () {
       doneCount++;
       if (doneCount == 2) controller.close();
     });
@@ skipping 307 matching lines @@
     error is FileIOException ||
     error is HttpException ||
     error is HttpParserException ||
     error is LinkIOException ||
     error is MimeMultipartException ||
     error is OSError ||
     error is ProcessException ||
     error is SocketIOException ||
     error is WebSocketException;
 }