getBoxDecoration method
Returns the BoxDecoration for the given key.
If key is null, the standard key will be used.
If key still is not found, null is returned.
Implementation
BoxDecoration? getBoxDecoration(Key? key) {
  key ??= _standardKey;
  if (decorationMap != null && decorationMap!.containsKey(key)) {
    return decorationMap![key];
  } else {
    return null;
  }
}