Miscellaneous small improvements for scheduled_test.

pkg/scheduled_test/lib/src/descriptor/directory_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 '../../../../../pkg/pathos/lib/path.dart' as path;
 
 import '../../descriptor.dart';
 import '../../scheduled_test.dart';
 import '../utils.dart';
 
 /// A path builder to ensure that [load] uses POSIX paths.
 final path.Builder _path = new path.Builder(style: path.Style.posix);
 
 /// A descriptor describing a directory containing multiple files.
 class DirectoryDescriptor extends Descriptor {
-  /// The entries contained within this directory.
-  final Iterable<Descriptor> contents;
+  /// The entries contained within this directory. This is intentionally
+  /// mutable.
+  final List<Descriptor> contents;
 
-  DirectoryDescriptor(String name, this.contents)
-      : super(name);
+  DirectoryDescriptor(String name, Iterable<Descriptor> contents)
+      : super(name),
+        contents = contents.toList();
 
   Future create([String parent]) => schedule(() {
     if (parent == null) parent = defaultRoot;
     var fullPath = path.join(parent, name);
     return new Directory(fullPath).create(recursive: true).then((_) {
       return Future.wait(
           contents.map((entry) => entry.create(fullPath)).toList());
     });
   }, 'creating directory:\n${describe()}');
 
@@ skipping 53 matching lines @@
           .replaceFirst('|   ', '|-- ');
       buffer.writeln(entryString);
     }
 
     var lastEntryString = prefixLines(contents.last.describe(), prefix: '    ')
         .replaceFirst('    ', "'-- ");
     buffer.write(lastEntryString);
     return buffer.toString();
   }
 }