Narrowing via assignment in switch broken by #35000

[sandersn]

#35000 breaks activex-scripting on Definitely Typed because narrowing via assignment in switch no longer works:

type O = {
    a: number,
    b: number
}
type K = keyof O | 'c'
function f(o: O, k: K) {
    switch(k) {
        case 'c':
            k = 'a'
    }
    k === 'c' // should have error here
    return o[k] // and no error here
}

Actual behavior:
No error on k === 'c' and error on o[k].

Using if (k === 'c') { instead of switch (k) { case 'c': works correctly.