Spread breaks exact object types

[ForbesLindesay]

The following example:

/* @flow */

const x: {|a: number|} = {a: 10};
const y: {|a: number, b: number|} = {...x, b: 10};

Produces the error:

4: const y: {|a: number, b: number|} = {...x, b: 10};
                                       ^ object literal. Inexact type is incompatible with exact type
4: const y: {|a: number, b: number|} = {...x, b: 10};
            ^ exact type: object type

but it shouldn't produce an error because x is an exact object, so spreading it into an object literal should not prevent that object literal being exact.

[vkurchatkin]

Duplicate of #2405