Copy flow's promise then() interface into PromiseLike
Copy flow's promise then() interface into PromiseLike

This allows Flow promise types to be compatible with what AVA is expecting

https://github.com/facebook/flow/blob/c88262be2be0b94b04bfaaf5e52570856d0b87e3/lib/core.js#L589-L597

sindresorhus/ava,avajs/ava,avajs/ava