Add a nothing descriptor to scheduled_test.

pkg/scheduled_test/lib/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.
 
 /// A library for declaratively describing a filesystem structure, usually for
 /// the purpose of creating or validating it as part of a scheduled test.
 ///
 /// You can use [dir] and [file] to define a filesystem structure. Then, you can
 /// call [Entry.create] to schedule a task that will create that structure on
 /// the physical filesystem, or [Entry.validate] to schedule an assertion that
@@ skipping 53 matching lines @@
 
 import 'dart:async';
 
 import 'package:pathos/path.dart' as path;
 
 import 'scheduled_test.dart';
 import 'src/descriptor/async.dart';
 import 'src/descriptor/directory.dart';
 import 'src/descriptor/entry.dart';
 import 'src/descriptor/file.dart';
+import 'src/descriptor/nothing.dart';
 
 export 'src/descriptor/async.dart';
 export 'src/descriptor/directory.dart';
 export 'src/descriptor/entry.dart';
 export 'src/descriptor/file.dart';
+export 'src/descriptor/nothing.dart';
 
 /// The root path for descriptors. Top-level descriptors will be created and
 /// validated at this path. Defaults to the current working directory.
 ///
 /// If this is set to `null`, it will reset itself to the current working
 /// directory.
 String get defaultRoot => _defaultRoot == null ? path.current : _defaultRoot;
 set defaultRoot(String value) {
   _defaultRoot = value;
 }
 String _defaultRoot;
 
 /// Creates a new text [File] descriptor with [name] and [contents].
 File file(Pattern name, [String contents='']) => new File(name, contents);
 
 /// Creates a new binary [File] descriptor with [name] and [contents].
 File binaryFile(Pattern name, List<int> contents) =>
     new File.binary(name, contents);
 
 /// Creates a new [Directory] descriptor with [name] and [contents].
 Directory dir(Pattern name, [Iterable<Entry> contents]) =>
     new Directory(name, contents == null ? <Entry>[] : contents);
 
 /// Creates a new descriptor wrapping a [Future]. This descriptor forwards all
 /// asynchronous operations to the result of [future].
 Async async(Future<Entry> future) => new Async(future);
+
+/// Creates a new [Nothing] descriptor that asserts that no entry named [name]
+/// exists.
+Nothing nothing(Pattern name) => new Nothing(name);