Calculates result of getSourceName() one time, so reuse because of CU cache.
Prepares SAXParserFActory one time, so avoids expensive ClassLoader lookups.

Review at http://gwt-code-reviews.appspot.com/1438801

Review by: scottb@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10162 8db76d5a-ed1c-0410-87a9-c151d255dfc7