Add missing file

Author: sigurdm

test/link_descriptor.dart

@@ -0,0 +1,56 @@
+// Copyright (c) 2024, 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.
+
+import 'dart:io';
+
+import 'package:path/path.dart' as p;
+import 'package:test/test.dart';
+
+import 'descriptor.dart' as d;
+
+/// Describes a symlink.
+class LinkDescriptor extends d.Descriptor {
+  /// On windows symlinks to directories are distinct from symlinks to files.
+  final bool forceDirectory;
+  final String target;
+  LinkDescriptor(super.name, this.target, {this.forceDirectory = false});
+
+  @override
+  Future<void> create([String? parent]) async {
+    final path = p.join(parent ?? d.sandbox, name);
+    if (forceDirectory) {
+      if (Platform.isWindows) {
+        Process.runSync('cmd', ['/c', 'mklink', '/D', path, target]);
+      } else {
+        Link(path).createSync(target);
+      }
+    } else {
+      Link(path).createSync(target);
+    }
+  }
+
+  @override
+  String describe() {
+    return 'symlink at $name targeting $target';
+  }
+
+  @override
+  Future<void> validate([String? parent]) async {
+    final link = Link(p.join(parent ?? d.sandbox, name));
+    try {
+      final actualTarget = link.targetSync();
+      expect(
+        actualTarget,
+        target,
+        reason: 'Link doesn\'t point where expected.',
+      );
+    } on FileSystemException catch (e) {
+      fail('Could not read link at $name $e');
+    }
+  }
+}
+
+d.Descriptor link(String name, String target, {bool forceDirectory = false}) {
+  return LinkDescriptor(name, target, forceDirectory: forceDirectory);
+}