Add a filesystem descriptor library to scheduled_test.

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

Index: pkg/scheduled_test/lib/src/descriptor/entry.dart
diff --git a/pkg/scheduled_test/lib/src/descriptor/entry.dart b/pkg/scheduled_test/lib/src/descriptor/entry.dart
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+// 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.entry;
+
+import 'dart:async';
+
+import 'utils.dart';
+
+/// The base class for various declarative descriptions of filesystem entries.
+/// All asynchronous operations on descriptors are [schedule]d unless otherwise
+/// noted.
+abstract class Entry {
+  /// The name of this entry. For most operations, this must be a [String];
+  /// however, if the entry will only be used for validation, it may be a
+  /// non-[String] [Pattern]. In this case, there must be only one entry
+  /// matching it in the physical filesystem for validation to succeed.
+  final Pattern name;
+
+  Entry(this.name);
+
+  /// Schedules the creation of the described entry within the [parent]
+  /// directory. Returns a [Future] that completes after the creation is done.
+  ///
+  /// [parent] defaults to [defaultRoot].
+  Future create([String parent]);
+
+  /// Schedules the validation of the described entry. This validates that the
+  /// physical file system under [parent] contains an entry that matches the one
+  /// described by [this]. Returns a [Future] that completes to `null` if the
+  /// entry is valid, or throws an error if it failed.
+  ///
+  /// [parent] defaults to [defaultRoot].
+  Future validate([String parent]);
+
+  /// Treats [this] as an in-memory filesystem and returns a stream of the
+  /// contents of the child entry located at [path]. This only works if [this]
+  /// is a directory entry. This operation is not [schedule]d.
+  ///
+  /// All errors in loading the file will be passed through the returned
+  /// [Stream].
+  Stream<List<int>> load(String pathToLoad) => errorStream("Can't load "
+      "'$pathToLoad' from within $nameDescription: not a directory.");
+
+  /// Returns the contents of [this] as a stream. This only works if [this] is a
+  /// file entry. This operation is not [schedule]d.
+  ///
+  /// All errors in loading the file will be passed through the returned
+  /// [Stream].
+  Stream<List<int>> read();
+
+  /// Asserts that the name of the descriptor is a [String] and returns it.
+  String get stringName {
+    if (name is String) return name;
+    throw 'Pattern $nameDescription must be a string.';
+  }
+
+  /// Returns a human-readable description of [name], for error reporting. For
+  /// string names, this will just be the name in quotes; for regular
+  /// expressions, it will use JavaScript-style `/.../` notation.
+  String get nameDescription => describePattern(name);
+
+  /// Returns a detailed tree-style description of [this].
+  String describe();
+}