error in disjoint union refinement when accessing attribute via quotes, i.e. obj['attr']

here is the example from the docs:

```
// @flow
type Success = { success: true, value: boolean };
type Failed = { success: false, error: string };

type Response = Success | Failed;

function handleResponse(response: Response) {
if (response.success) {
var value: boolean = response.value; // Works!
} else {
var error: string = response.error; // Works!
}
}
```

it works. However if I try to access the attribute via quotes, not via dot notation, it breaks:

```
function handleResponse2(response: Response) {
if (response['success']) {
var value: boolean = response.value; // Works!
} else {
var error: string = response.error; // Works!
}
}
```

https://flow.org/try/#0PTAEAEDMBsHsHcBQAXAngBwKagMoFcBjAzAZxNAF5QBvUEw4sgLlGQCc9MAaUANwENonFgCNYsaJn4A7UAF8A3CgzYAYvwCWkgCahKNOg1IkWkQSW6hMbNrDYsS7DdIDm8pcqygASqXSxpC318ImNQAB9QdS1MbQ9IPGkCZA0A0AALGW1JXxJ-QMwACjY-AIsWXPyLAEoaRD0NSFBi0oKAOnpQslrqer0+fjYBoUxRcUkZfRK8ssw2gRGFUBBQAHU7AGsSAEI+uStoIN7+gaHrW3s6J1cp1os287sllfW2Ld29OUQvxEQEpJSaUy0mymEqswATC0ZgUKndMD0+o1mtMqpgANoAck6jBImIAuoiTgIhgthKAxBIpLIqKjZvNBJxnmBXu89gcjn09CSrDY7A5rm5afCHny2My1psdntvkA