IndexedAccess still drops index signature on ReadonlyArray

[ajafff]

TypeScript Version: 3.5.0-dev.20190427

Follow-up on #30938 @ahejlsberg

Code

function fn<T extends ReadonlyArray<string>>(param: T, cb: (element: T[number]) => void) {
    cb(param[0]); // Argument of type 'string' is not assignable to parameter of type 'T[number]'.
}

Expected behavior:
Works with Array<string> and ReadonlyArray<string>.

Actual behavior:
Fails with ReadonlyArray.

Related Issues:
#30938
#31000