We can't use a classes object into field as classes is already defined and is a function. What do you think about "cssClasses" ?
Also, I'm embarrassed because 'label' is a JavaScript keyword.
```javascript
var aForm = forms.create({
  name: fields.string({
    required: true,
    label: 'fieldname',
    cssClasses: {
      field: ['an-extra-wrapper-class', 'another-extra-wrapper-class'],
      label: ['an-extra-label-class']
    }
  })
});
```