Value accessors

[jaripekkala]

final data = {
  'product': {
    'id': 2,
    'name': 'Foo',
  },
  'product_id': 2,
};

final form = FormGroup({
  'product_id': FormControl(data['product'], [Required()]),
});

assert(form.value != {'product_id': 2});

Use cases: autocompletes with initial value, single stored values controlled via multiple form controls (date & time fields separate however stored as datetimes)