1.8: type guard no longer narrows from {} to primitive

[jeffreymorlan]

This works in 1.7:

var a: {};
if (typeof a === 'number') {
    var b: number = a;
}

but in the master branch, a is not narrowed and you get an error. This seems to have started with commit febda00

[DanielRosenwasser]

👏 👏 @weswigham