FIX: fixed bug with useFlowReadOnlyTypes on Enums

[zanettin]

just fixing a bug which occurred after introducing useFlowReadOnlyTypes on handling Enums.

current output

export const MyEnumValues = Object.freeze({
  +A: 'A',
  +B: 'B',
  +C: 'C'
});

fixed output

export const MyEnumValues = Object.freeze({
  A: 'A',
  B: 'B',
  C: 'C'
});

[dotansimha]

Nice, I'm merging, but I'll need to reimplement it here again: #1353, because TypeScript plugins and Flow plugins have common logic now, and we refactored them.
Thanks @zanettin !

[zanettin]

thanks a lot @dotansimha 🎉