Spread operator on a function should not be allowed

[singpolyma-shopify]

This code:

// @flow
const x = [];
const y : Array<{ hello: string }> = [...x.concat, { hello: "there" }];

Passes flow currently, however the result of ...x.concat is undefined which results in a value being assigned to y that violates the type.

[vkurchatkin]

This is caused by #106