The marshalling backend has been partially verified in this playground project https://github.com/evacchi/gwt-bpmn-marshallers
I have started working on the submarine stand-alone showcase to use these marshallers. To try the result, follow these instructions: https://gist.github.com/romartin/96c200428a140e026010fad6e4172570

By following these steps, you should be able to compile under GWT successfully.

The BPMN marshaller provided with https://github.com/evacchi/gwt-bpmn-marshallers must be installed. However, these marshallers might not be strictly required if we provide a source package for the marshallers we already use. This is already working for drools and bpsim namespaces (see JBPM-8546)

However, the demo is not complete. This task can be considered *done* when an end-to-end unmarshalling demo (from BPMN source to diagram) is possible.

It seems that reading XML from GWT DOM into Eclipse BPMN models works (e.g. paste any BPMN file in the textarea, see the toString() of the results in the right-hand side area)

In order for this task to be complete, you should be able to run these lines:
https://github.com/evacchi/kie-wb-common/blob/b93a9d982f34f02764b0a8c42ba76fd6d17855c5/kie-wb-common-stunner/kie-wb-common-stunner-showcase/kie-wb-common-stunner-showcase-standalone/src/main/java/org/kie/workbench/common/stunner/standalone/client/services/BPMNStandaloneClientDiagramServiceImpl.java#L109-L163