Narrow the parent type of a singleton type to never in the false branch

If the type of the member checked is not falsy (and so is never in the false branch).

TypeScript Version: 2.7.0-dev.201xxxxx

Code

const x: {a: 1, b: 2} = { a: 1, b: 2 };
if (x.a) {

}
else {
    x.a;
}

Expected behavior:
type of x in the else branch is never.

Actual behavior:
type of x is {a: 1. b: 2}, and x.a is (counterintuitively) never.

This would bring the behavior of truthiness guards in line with our new behavior for in type guards.

Comienza reproduciendo el ejemplo proporcionado con el TypeScript compiler, preferiblemente contra typescript@next, tal como se solicita. Rastrea el comportamiento de narrowing del truthiness guard y compáralo con el comportamiento indicado del type guard in; se considera completado cuando x se reduce a never en la rama falsa, preservando al mismo tiempo el tipo esperado de x.a.