code: 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);
  }
docs: Concatenate each extern as a variable declaration that points to an empty object.
  Externs are used to prevent the closure compiler from complaining about undeclared
  variables and renaming those variables.
  
  @function writeExterns
  @param {Object} grunt
  @param {Object} fileData
  @param {Object} options
  @api private