[DMN Designer] Marshaller: Conversion from 1.1 to 1.2 leads to 'Unable to resolve type reference' errors

Following the discussion on https://issues.jboss.org/browse/DROOLS-3038?focusedCommentId=13654791&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13654791 (opening a 1.1 file, saving, leading to lots of 'Unable to resolve type reference' errors for built-in types.

On investigation it appears a lot of our backend marshaller tests are also reporting the error following conversion from 1.1 to 1.2.