Expand() usage does not pickup .output if task with expand comes before in yaml def/dict keys of task referenced in expand

[StephenOTT]

If you have something like

Tasks:
   Task1:
      ...
      Dependencies: [Task2]
      Expand: 
         Some_arg: Task2.output

   Task2:
      ...

Then when dagbuilder parses the tank/duct is does not properly change the expand arg to a referenced Xcom and it remains as a are

[tatiana]

Thanks a lot for reporting this issue, @StephenOTT ! Would you be willing to contribute a fix? We should add an example DAG that leverages this so we can avoid regressions, once we fix this.