Revert "Remaining private libs"

sdk/lib/async/async_error.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.
 
-part of "dart:async";
+part of "async.dart";
 
 _invokeErrorHandler(
     Function errorHandler, Object error, StackTrace stackTrace) {
   if (errorHandler is ZoneBinaryCallback<dynamic, Null, Null>) {
     return (errorHandler as dynamic)(error, stackTrace);
   } else {
     ZoneUnaryCallback unaryErrorHandler = errorHandler;
     return unaryErrorHandler(error);
   }
 }
 
 Function _registerErrorHandler<R>(Function errorHandler, Zone zone) {
   if (errorHandler is ZoneBinaryCallback<dynamic, Null, Null>) {
     return zone.registerBinaryCallback<R, Object, StackTrace>(
         errorHandler as dynamic/*=ZoneBinaryCallback<R, Object, StackTrace>*/);
   } else {
     return zone.registerUnaryCallback<R, Object>(
         errorHandler as dynamic/*=ZoneUnaryCallback<R, Object>*/);
   }
 }