Asynchronous Field-Level Validation Example does not work properly

### Are you submitting a **bug report** or a **feature request**?

Bug report

### What is the current behavior?

_STEPS TO REPRODUCE:_

- Access: [Asynchronous Field-Level Validation Example](https://codesandbox.io/s/wy7z7q5zx5)
- Click on the submit button/focus on the `Username` field and unfocus then.

Still able to submit without the `Username` value.

### What is the expected behavior?

The `Required` validation text should be shown following the code:

```
const usernameAvailable = simpleMemoize(async value => {
if (!value) {
return "Required";
}
...
```

### Sandbox Link

https://codesandbox.io/s/wy7z7q5zx5