_slash(filePath) {
    if (os.platform() === 'win32') {
      filePath = filePath.replace(/\\/g, '/');
    }

    return filePath;
  }
convert 'back slash' to 'slash'.
path separator is 'back slash' if platform is windows.
@param {string} filePath - target file path.
@returns {string} converted path.
@private