Description of problem:
If you will add new Swimlane and try to add to this Swimlane some element (which is already exist in process, or element which is added after Swimlane but out of Swimlane and then moved to Swimlane) Swimlane source will have no associated elements. See attachment.

Version-Release number of selected component (if applicable):
6.1.3.CR1

How reproducible:
always

Steps to Reproduce:
1. Create Business Process
2. Add some task [first task]
3. Add Swimlane
4. Add new task [second task] to Swimlane
5. See source of Swimlane
6. Move Start Event and first task to Swimlane
7. See source of Swimlane
8. Add task to canvas (beside the Swimlane)
9. Move task to Swimlane
10. See source of Swimlane

Actual results:
On point 7 and 10 no new element in Swimlane source appear
On point 6 Start Event and First Task will disappear (will be under the Swimlane)

Expected results:
All element will be associate with Swimlane in any browser.

Additional info:
Server warning when trying to see Swimlane source: 13:29:44,289 WARN [org.jbpm.designer.bpmn2.impl.Bpmn2JsonUnmarshaller] (http-localhost/127.0.0.1:8080-25) index=0, size=0

Affected: FF31esr, FF24esr, Google Chrome, IE10, IE11
NOT Affected: FF40