`class="fa fa-eye"` comes with the [font-awesome.css](https://github.com/cvan/galaxy/blob/1f8f811d0fe8105b24282df4a4ab122f9fab5b65/src/media/css/font-awesome.css) and the CSS for the `toggle-preview` is [here](https://github.com/cvan/galaxy/blob/1f8f811d0fe8105b24282df4a4ab122f9fab5b65/src/media/css/forms.styl#L333-L340).