function message() {
      if (options.passed) {
        return 'Passed.';
      } else if (options.message) {
        return options.message;
      } else if (options.error) {
        return messageFormatter(options.error);
      }
      return '';
    }
@typedef Expectation
@property {String} matcherName - The name of the matcher that was executed for this expectation.
@property {String} message - The failure message for the expectation.
@property {String} stack - The stack trace for the failure if available.
@property {Boolean} passed - Whether the expectation passed or failed.
@property {Object} expected - If the expectation failed, what was the expected value.
@property {Object} actual - If the expectation failed, what actual value was produced.