'instanceof' narrowing works awkwardly with empty object types & primitives

[DanielRosenwasser]

Try in the playground.

class Dog {
}

declare let x: Dog | string;

if (x instanceof Dog) {
    x
}
else {
    x.toLowerCase()
}

Expected: No error, first branch's x has type Dog, second branch's x has type string
Actual: Property 'toLowerCase' does not exist on type 'never'. (first branch is string | Dog, second is never)

We should be accounting for primitives a little less sloppily. instanceof checks don't work on primitive types.

[DanielRosenwasser]

Specifically, any changes here should ensure that

function foo(x: number | string) {
    if (x instanceof Number) {
        x
    }
}

x is not narrowed (or is narrowed to never) in the above example.

[DanielRosenwasser]

Actually, we should just issue an error when no constituent of a union is an object type.