code: function assertion(keys) {
    var missing = keys.filter(isMissing);
  
    if (missing.length) {
      throw new ReferenceError(format(message, missing.join(', ')));
    }
  }
docs: Assert that the given keys are found within `process.env`.
  @throws ReferenceError
  
  @param {Array} keys
  keys that are to be found within `process.env`.