assert is not typechecked as AssertError

This works:
```
a = match x {
  | F => v
  | _ => throw AssertError
}
```

This doesn't work
```
a = match x {
  | F = v
  | _ = assert(false)
}
```
```
error: the appropriate match for 'assert' of type '(bool -> CLine.arg@12)' is not found.
```