Change the way Patterns work in scheduled_test/descriptor.

pkg/scheduled_test/lib/src/descriptor/nothing_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';
+
+import '../../../../../pkg/pathos/lib/path.dart' as path;
+
+import '../../descriptor.dart';
+import '../../scheduled_test.dart';
+import '../utils.dart';
+
+/// A descriptor that validates that no file exists with the given name.
+/// Creating this descriptor is a no-op and loading from it is invalid.
+class NothingDescriptor extends Descriptor {
+  NothingDescriptor(String name)
+      : super(name);
+
+  Future create([String parent]) => new Future.immediate(null);
+
+  Future validate([String parent]) => schedule(() => validateNow(parent),
+      "validating '$name' doesn't exist");
+
+  Future validateNow([String parent]) => new Future.of(() {
+    if (parent == null) parent = defaultRoot;
+    var fullPath = path.join(parent, name);
+    if (new File(fullPath).existsSync()) {
+      throw "Expected nothing to exist at '$fullPath', but found a file.";
+    } else if (new Directory(fullPath).existsSync()) {
+      throw "Expected nothing to exist at '$fullPath', but found a "
+          "directory.";
+    } else {
+      return;
+    }
+  });
+
+  Stream<List<int>> load(String pathToLoad) => errorStream("Nothing "
+      "descriptors don't support load().");
+
+  Stream<List<int>> read() => errorStream("Nothing descriptors don't support "
+      "read().");
+
+  String describe() => "nothing at '$name'";
+}