Make a bunch of stuff in pub synchronous.

utils/pub/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';
+
 /// An [ErrorGroup] entangles the errors of multiple [Future]s and [Stream]s
 /// with one another. This allows APIs to expose multiple [Future]s and
 /// [Stream]s that have identical error conditions without forcing API consumers
 /// to attach error handling to objects they don't care about.
 ///
 /// To use an [ErrorGroup], register [Future]s and [Stream]s with it using
 /// [registerFuture] and [registerStream]. These methods return wrapped versions
 /// of the [Future]s and [Stream]s, which should then be used in place of the
 /// originals. For example:
 ///
@@ skipping 234 matching lines @@
         onDone: onDone,
         unsubscribeOnError: true);
   }
 
   /// Signal that an error from [_group] should be propagated through [this],
   /// unless it's already complete.
   void _signalError(AsyncError e) {
     if (_isDone) return;
     _subscription.cancel();
     // Call these asynchronously to work around issue 7913.
-    new Future.immediate(null).then((_) {
+    defer(() {
       _controller.signalError(e.error, e.stackTrace);
       _controller.close();
     });
   }
 }