docs: function writeExterns (grunt, fileData, options) {
      let content = '';
  
      // Default externs.
      content += 'var require = function () {};';
  
      for (let i = 0, max = options.externs.length; i < max; i++) {
          content += `var ${options.externs[i]} = {};`;
      }
  
      grunt.file.write(fileData.tmpExterns, content);
  }