code: function (actual, message) {
          YUITest.Assert._increment();
          if (typeof actual != "boolean"){
              throw new YUITest.UnexpectedValue(YUITest.Assert._formatMessage(message, "Value should be a Boolean."), actual);
          }
      }
docs: Asserts that a value is a Boolean.
  @param {Object} actual The value to test.
  @param {String} message (Optional) The message to display if the assertion fails.
  @method isBoolean
  @static