RFR: 8160435: Source.baseURL is slow for URLs with unregistered protocol

[Hannes Wallnöfer]

Please review:

Bug: https://bugs.openjdk.java.net/browse/JDK-8160435
Webrev: http://cr.openjdk.java.net/~hannesw/8160435/webrev.00/

This cleans up Source.baseURL() and avoids the potential performance problem of 
URL constructor throwing MalformedURLException.

Thanks,
Hannes