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