`exit` flow directive isn't validated properly

[JBBianchi]

What happened:
When trying to use the exit flow directive, the validation process of the SDK returns an error.

What you expected to happen:
The exit flow directive to be valid.

How to reproduce it:

• Try to validate the following example from the spec:

document:
  dsl: '1.0.0-alpha5'
  namespace: test
  name: sample-workflow
  version: 0.1.0
do:
  - processOrder:
      switch:
        - case1:
            when: .orderType == "electronic"
            then: processElectronicOrder
        - case2:
            when: .orderType == "physical"
            then: processPhysicalOrder
        - default:
            then: handleUnknownOrderType
  - processElectronicOrder:
      do:
        - validatePayment:
            set:
              validate: true
        - fulfillOrder:
            set:
              status: fulfilled
      then: exit
  - processPhysicalOrder:
      do:
        - checkInventory:
            set:
              inventory: clear
        - packItems:
            set:
              items: 1
        - scheduleShipping:
            set:
              address: Elmer St
      then: exit
  - handleUnknownOrderType:
      do:
        - logWarning:
            set:
              log: warn
        - notifyAdmin:
            set:
              message: something's wrong

Which results in the following errors:

The workflow definition is not valid.

/do[1]/processElectronicOrder/then:
Unreachable or undefined task
/do[2]/processPhysicalOrder/then:
Unreachable or undefined task