Replace defer() with Future.of in pub.

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';
 
@@ skipping 246 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.
-    defer(() {
+    new Future.immediate(null).then((_) {
       _controller.addError(e.error, e.stackTrace);
       _controller.close();
     });
   }
 }