anchor.diff:  }
   
   abstract class DirectoryPopulater {
  -  Future populate(Directory dir);
  +  Future<Directory> populate(Directory dir);
   }
   
   class MapDirectoryPopulater extends DirectoryPopulater {
  
positive.code: 
    MapDirectoryPopulater(this._contents);
  
    Future populate(Directory dir) {
      assert(dir != null);
      return IoHelpers.isEmpty(dir)
          .chain((bool isEmpty) {
  
negative.code:     return file.writeAsString(content);
    }
  
    static Future<Directory> _createDirAndPopulate(Directory parent, String name, Map<String, dynamic> content) {
      final subDirPath = new Path(parent.path).append(name);
      final subDir = new Directory.fromPath(subDirPath);
      assert(!subDir.existsSync());
  