code: function sha256stream() {
    var sha256 = createHash('sha256');
    var stream = new Transform();
    stream._transform = function(chunk, encoding, callback) {
      this.push(chunk);
      sha256.update(chunk);
      callback();
    };
    stream.on('finish', () => {
      stream.emit('sha256', sha256.digest('hex'));
    });
    return stream;
  }
docs: /* istanbul ignore next