This syntax allows you to specify whether a type variable can appear in
a covariant or contravariant position, and is super useful for, say,
Promise.
Right now this is hacked in jankily, but in the next major release we
should stop using Identifier nodes for type parameters.
An interface export is just like a type export. In fact, it's a syntax affordance which makes the following equivalent:
```javascript
interface I_ { ... }
export type I = I_;
```
```javascript
export interface I { ... }
```
See facebook/flow#1145
This has been a feature in Flow for a long time (couldn't easily find a
specific commit adding this). Interfaces are basically undocumented, though, so
it's easy to see how this was missed.
