Reproduce steps:

• Add a message throw event to a process diagram
• Mark the message throw event as async (before or after doesn't matter)
• Save the diagram and open Source view

Problem:

• Async attribute is placed within the messageEventDefinition, see:

      <bpmn2:intermediateThrowEvent id="IntermediateThrowEvent_1">
        <bpmn2:incoming>SequenceFlow_1</bpmn2:incoming>
        <bpmn2:outgoing>SequenceFlow_3</bpmn2:outgoing>
        <bpmn2:messageEventDefinition id="_MessageEventDefinition_2" camunda:async="true"/>
      </bpmn2:intermediateThrowEvent>
  

Expected behavior:

• Should be:

      <bpmn2:intermediateThrowEvent id="IntermediateThrowEvent_1" camunda:async="true">
        <bpmn2:incoming>SequenceFlow_1</bpmn2:incoming>
        <bpmn2:outgoing>SequenceFlow_3</bpmn2:outgoing>
        <bpmn2:messageEventDefinition id="_MessageEventDefinition_2"/>
      </bpmn2:intermediateThrowEvent>