If you are using ES6 class syntax, then I would think you would also be using const / let; and if you are using const / let, then FlowTypes is, for me anyway, borderline unusable because it still does not support them (although there is apparently a fork with pseudo-support, if you want to deal with that):

https://github.com/facebook/flow/pull/431

Why the heck would you not want all that juicy ES6 ES2015 goodness?

No one stops you to use ES6 notation in React.createComponent methods.