UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/69.0.3497.92 Safari/537.36

Steps to reproduce the problem:
1. listen to `keydown` event
2. call `preventDefault()`
3. alt+i always inserts ^ even if it should not

What is the expected behavior?
alt+i should not insert ^ if I call `preventDefault()`

What went wrong?
^ is inserted

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 69.0.3497.92  Channel: n/a
OS Version: OS X 10.13.6
Flash Version: 

Easy repro https://jsfiddle.net/rnL03gty/2/

```
var input = document.getElementById('in');
var output = document.getElementById('out');
input.focus();
input.addEventListener('keydown', function(e) {
  if (e.altKey && e.code === 'KeyI') {
  	e.preventDefault();
    output.innerHTML += 'preventing default!<br/>';
  }
});
```

Extracted from https://github.com/Microsoft/vscode/issues/41024