Align flow-parser with babylon v7

[danez]

Hi,

we are getting close to release a beta of v7 of babel soon.

We changed some things in v7 in coordination with you guys, and this issue is a reminder to integrate this changes into flow:

Create separate Variance Node

As coordinated with @samwgoldman the variance should be represented as separate node, which is already done in babel v7. Flow should be updated accordingly
Related tickets: babel/babel#4722, babel/babel#4697, babel/babylon#161, babel/babylon#333

Add method property to FunctionTypeAnnotation node

This was discussed and seems agreed on. babylon has an open PR for this: babel/babylon/334
Related tickets: babel/babylon#331

I would create PRs for this myself, but well OCaml :)

Thanks.

//cc @samwgoldman @mroch

[mroch]

Method property added in 622bbc4

@danez It looks like babel/babylon#331 became babel/babel#6704 and is still open. its PR was closed without being merged.

[danez]

Thanks

[samwgoldman]

I have a change pending review internally for the variance node. Just waiting on @mroch to review :)