The icon of the item.
If activeIcon is provided, this will only be displayed when the item is not selected.
To make the bottom navigation bar more accessible, consider choosing an icon with a stroked and filled version, such as Icons.cloud and Icons.cloud_queue. icon should be set to the stroked version and activeIcon to the filled version.
If a particular icon doesn't have a stroked or filled version, then don't pair unrelated icons. Instead, make sure to use a BottomNavigationBarType.shifting.
final Widget icon;