I maintain PayPal's cross domain suite of libraries [0] and I'm fully intending to drop Flow for TS. The single thing I'm waiting for is https://github.com/Microsoft/TypeScript/issues/21699 since we use a fair amount of custom jsx rendering [1]. That's the one main thing (for me) that Flow is way stronger at right now.

[0]: https://medium.com/@bluepnume/introducing-paypals-open-sourc...

[1]: https://medium.com/@bluepnume/jsx-is-a-stellar-invention-eve...