code: function assertOut(key, expected, result) {
    var actual = result[key];
    var statement = expected instanceof RegExp
      ? expected.test(actual)
      : expected === actual;
  
    if (statement !== true) {
      var message = 'Expected ' + key +' to match "' + expected + '". Actual: "' + actual + '"';
      return error(result, message, expected, actual);
    }
  }
docs: Assert stdout or stderr.
  
  @param {String} stdout/stderr
  @param {Mixed} expected
  @param {Result} result
  @returns {AssertionError|null}
  @api private