code: function unifiedDiff (err) {
    let indent = '      '
    function cleanUp (line) {
      if (line[0] === '+') {
        return indent + format.colorLines('green', line)
      }
      if (line[0] === '-') {
        return indent + format.colorLines('red', line)
      }
      if (line.match(/@@/)) {
        return null
      }
      if (line.match(/\\ No newline/)) {
        return null
      }
      if (line.trim().length) {
        line = format.colorLines('white', line)
      }
      return indent + line
    }
    function notBlank (line) {
      return typeof line !== 'undefined' && line !== null
    }
    let msg = diff.createPatch('string', err.actual, err.expected)
    let lines = msg.split('\n').splice(4)
    let diffResult = lines.map(cleanUp).filter(notBlank).join('\n')
    if (!diffResult.trim().length) {
      msg = diff.createPatch(
        'string',
        stringify(Object.keys(err._actual || err.actual).sort()),
        stringify(Object.keys(err._expected || err.expected).sort())
      )
      lines = msg.split('\n').splice(4)
      diffResult = format.red('       object keys not match: \n') + lines.map(cleanUp).filter(notBlank).join('\n')
    }
    return '\n      ' +
      format.colorLines('green', '+ expected') + ' ' +
      format.colorLines('red', '- actual') +
      '\n\n' +
      diffResult
  }
docs: Returns a unified diff between two strings.
  @private
  @param {Error} err with actual/expected
  @return {string} The diff.