Allow DirectoryDescriptor.load to load a file next to a directory.

pkg/scheduled_test/lib/src/descriptor/file_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.file;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:math' as math;
 import 'dart:utf';
 
 import 'package:pathos/path.dart' as path;
 
 import '../../descriptor.dart';
 import '../../scheduled_test.dart';
 import '../utils.dart';
 
 /// A descriptor describing a single file.
-class FileDescriptor extends Descriptor {
+class FileDescriptor extends Descriptor implements ReadableDescriptor {
   /// The contents of the file, in bytes.
   final List<int> contents;
 
   /// The contents of the file as a String. Assumes UTF-8 encoding.
   String get textContents => new String.fromCharCodes(contents);
 
   /// Creates a new text [FileDescriptor] with [name] that matches its String
   /// contents against [matcher]. If the file is created, it's considered to be
   /// empty.
   factory FileDescriptor.matcher(String name, Matcher matcher) =>
@@ skipping 110 matching lines @@
 
   _MatcherFileDescriptor(String name, this._matcher, {bool isBinary})
       : _isBinary = isBinary == true ? true : false,
         super._(name, <int>[]);
 
   void _validateNow(List<int> actualContents) =>
       expect(
           _isBinary ? actualContents : new String.fromCharCodes(actualContents),
           _matcher);
 }