code: function getIndentText (firstToken) {
      const text = sourceCode.text
      let i = firstToken.range[0] - 1
  
      while (i >= 0 && !LT_CHAR.test(text[i])) {
        i -= 1
      }
  
      return text.slice(i + 1, firstToken.range[0])
    }
docs: Get the text of the indentation part of the line which the given token is on.
  @param {Token} firstToken The first token on a line.
  @returns {string} The text of indentation part.