Typo: NoAlertPresentException vs. NoAlertOpenException

[seongjaelee]

https://github.com/google/webdriver.dart/blob/master/lib/src/target_locator.dart says:

/// Throws [NoAlertPresentException] if there is not currently an alert.

However,
https://github.com/google/webdriver.dart/blob/master/lib/src/exception.dart says:

        case 27: // NoAlertOpenError
          return new NoAlertOpenException(status, message);

Java WebDriver exception name is NoAlertPresentException.
http://selenium.googlecode.com/git/docs/api/java/org/openqa/selenium/NoAlertPresentException.html

I prefer to use NoAlertPresentException, because it is also used in Java version. However, if you want to stick to use NoAlertOpenException, please fix the comment. Thanks!