"property `prototype` is missing in statics of ..."

Please check the following code:

```
class BaseClass {
}

const checkIfCorrectClass___WORKS = (classToCheck: Class):boolean => {
return classToCheck.prototype instanceof BaseClass;
};

const checkIfCorrectClass___DOES_NOT_WORK = (classToCheck: Class): void => {
if (classToCheck.prototype instanceof BaseClass) {
return;
}

throw Error(`Invalid class received.`);
};
```
Receiving following error:

Here's the snippet:

https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVBjGBDAzrsAITwFMBhHfMAb1QF90M4A7XAFzAwAsSMBrAJJQycAE6jebCnlwB9AOoB5AEoBpAMpgAvGAAUWGQBU4ZHvwBcYafgA8xXOUq4AfAEpzAIzhwYJbM21nGlQwULAJNgBXUQCDfGNTXj4AOgAHUTg2TIBPVJIwAEtWNn8MEjgoIlJrXABuenrMFnZOM0FhMQkMKSdZABFFAFF1WQA5RUMFFVVtPTjcBLbLGrtqpzdLADc4AoATQOCwwsr9J0WktIystlz8ovZS8sr7RxlXQ6OwiOjmeqOGEJhNhcDJIQbiMS6AAGAmYm2wMD2nCc4V4JAKmxIu2SUNc9TotSAA

Could this be a bug maybe?