Make new StreamController be async by default.

sdk/lib/_internal/pub/lib/src/error_group.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 error_group;
 
 import 'dart:async';
 
 import 'utils.dart';
 
@@ skipping 221 matching lines @@
   /// The [StreamSubscription] that connects the wrapped [Stream] to
   /// [_controller].
   StreamSubscription _subscription;
 
   /// Whether [this] has any listeners.
   bool get _hasListeners => _controller.hasListener;
 
   /// Creates a new [_ErrorGroupFuture] that's a child of [_group] and wraps
   /// [inner].
   _ErrorGroupStream(this._group, Stream inner)
-    : _controller = new StreamController() {
+    : _controller = new StreamController(sync: true) {
     this._stream = inner.isBroadcast
         ? _controller.stream.asBroadcastStream()
         : _controller.stream;
     _subscription = inner.listen((v) {
       _controller.add(v);
     }, onError: (e) {
       _group._signalError(e);
     }, onDone: () {
       _isDone = true;
       _group._signalStreamComplete(this);
@@ skipping 15 matching lines @@
   void _signalError(var e) {
     if (_isDone) return;
     _subscription.cancel();
     // Call these asynchronously to work around issue 7913.
     new Future.value().then((_) {
       _controller.addError(e);
       _controller.close();
     });
   }
 }