Allow DirectoryDescriptor.load to load a file next to a directory.

pkg/scheduled_test/lib/src/descriptor/async_descriptor.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 descriptor.async;
 
 import 'dart:async';
 import 'dart:io' as io;
 
 import '../../descriptor.dart';
@@ skipping 15 matching lines @@
       : super('<async descriptor>');
 
   Future create([String parent]) =>
     schedule(() => future.then((entry) => entry.create(parent)));
 
   Future validate([String parent]) => schedule(() => validateNow(parent));
 
   Future validateNow([String parent]) =>
     future.then((entry) => entry.validateNow(parent));
 
-  Stream<List<int>> load(String path) => errorStream("AsyncDescriptors don't "
-      "support load().");
-
-  Stream<List<int>> read() => errorStream("AsyncDescriptors don't support "
-      "read().");
-
   String describe() => "async descriptor";
 }