readFile(filename) {
    const fs = this.fs;
    return new Promise((resolve, reject) => {
      const callback = (err, data) => {
        if (err) reject(err);
        else resolve(data);
      };
      if (fs && fs.readFile) {
        fs.readFile(filename, callback);
      } else {
        readFile(filename, 'utf8', callback);
      }
    });
  }
Read the contents of a file from the specified filesystem or disk