Object value types are unioned when computed property names are used

[zakjan]

TypeScript Version: 2.2.2

Code

const a = "a";
const b = "b";

const x = {
    [a]: "aaa",
    [b]: 123,
};

const y = {
    a: "aaa",
    b: 123,
};

Expected behavior:
both x, y have type {a: string, b: number}

Actual behavior:
x has type {[x: string]: string | number}
y has type {a: string, b: number}

[mhegazy]

Same underlying cause as #5579