If a web start main-class is loaded from a jar in the Trusted-Library class loader the application will fail with a exception.
This is caused by a specific check added in 6u18 after the main class is loaded
to ensure that it is defined in either the primary JNLPClassLoader or in the
JNLPPreverifyClassLoader added in 6u18. The fix is to also allow it to be
defined in the Trusted-Library loader added in 6u19.