Add a filesystem descriptor library to scheduled_test.

pkg/scheduled_test/lib/src/descriptor/utils.dart

Index: pkg/scheduled_test/lib/src/descriptor/utils.dart
diff --git a/pkg/scheduled_test/lib/src/descriptor/utils.dart b/pkg/scheduled_test/lib/src/descriptor/utils.dart
new file mode 100644
index 0000000000000000000000000000000000000000..f0d594cc429e5a6787bca9f68c81ce67c4c8cd33
--- /dev/null
+++ b/pkg/scheduled_test/lib/src/descriptor/utils.dart
@@ -0,0 +1,53 @@
+// 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.utils;
+
+import 'dart:io';
+
+import 'package:pathos/path.dart' as path;
+
+/// Returns a single filesystem entry within [parent] whose name matches
+/// [pattern]. If [pattern] is a string, looks for an exact match; otherwise,
+/// looks for an entry that contains [pattern].
+///
+/// If there are no entries in [parent] matching [pattern], or more than one,
+/// this will throw an exception.
+///
+/// [type] is used for error reporting. It should be capitalized.
+String entryMatchingPattern(String type, String parent, Pattern pattern) {
+  if (pattern is String) {
+    var path = path.join(parent, pattern);
+    if (new File(path).existsSync() || new Directory(path).existsSync()) {
+      return path;
+    }
+    throw "$type not found: '$path'.";
+  }
+
+  var matchingEntries = new Directory(parent).listSync()
+      .map((entry) => entry is File ? entry.fullPathSync() : entry.path)
+      .where((entry) => entry.contains(pattern))
+      .toList();
+
+  if (matchingEntries.length == 0) {
+    throw "No entry found in '$parent' matching ${describePattern(pattern)}.";
+  } else if (matchingEntries.length > 1) {
+    throw "Multiple entries found in '$parent' matching "
+          "${describePattern(pattern)}:\n"
+      "${matchingEntries.map((entry) => '* $entry').join('\n')}";
+  } else {
+    return matchingEntries.first;
+  }
+}
+
+/// Returns a human-readable description of [pattern].
+String describePattern(Pattern pattern) {
+  if (pattern is String) return "'$pattern'";
+  if (pattern is! RegExp) return '$pattern';
+
+  var flags = new StringBuffer();
+  if (!pattern.isCaseSensitive) flags.add('i');
+  if (pattern.isMultiLine) flags.add('m');
+  return '/${pattern.pattern}/$flags';
+}