Add a nothing descriptor to scheduled_test.

pkg/scheduled_test/lib/src/descriptor/nothing.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 'package:pathos/path.dart' as path;
+
+import '../../descriptor.dart' as descriptor;
+import '../../scheduled_test.dart';
+import '../utils.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 Nothing extends descriptor.Entry {
+  Nothing(Pattern name)
+      : super(name);
+
+  Future create([String parent]) => new Future.immediate(null);
+
+  Future validate([String parent]) => schedule(() {
+    if (parent == null) parent = descriptor.defaultRoot;
+    if (name is String) {
+      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;
+      }
+    }
+
+    return new Directory(parent).list().toList().then((entries) {
+      var matchingEntries = entries
+          .map((entry) => entry is File ? entry.fullPathSync() : entry.path)
+          .where((entry) => path.basename(entry).contains(name))
+          .toList();
+      matchingEntries.sort();
+
+      if (matchingEntries.length == 0) return;
+      throw "Expected nothing to exist in '$parent' matching $nameDescription, "
+              "but found:\n"
+          "${matchingEntries.map((entry) => '* $entry').join('\n')}";
+    });
+  }, "validating $nameDescription doesn't exist");
+
+  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 $nameDescription";
+}