Add const keyword where applicable

Author: michalsrutek

lib/date_widget.dart

@@ -33,13 +33,13 @@ class DateWidget extends StatelessWidget {
  return InkWell(
  child: Container(
  width: width,
- margin: EdgeInsets.all(3.0),
+ margin: const EdgeInsets.all(3.0),
  decoration: BoxDecoration(
- borderRadius: BorderRadius.all(Radius.circular(8.0)),
+ borderRadius: const BorderRadius.all(Radius.circular(8.0)),
  color: selectionColor,
  ),
  child: Padding(
- padding: EdgeInsets.all(8),
+ padding: const EdgeInsets.all(8),
  child: Column(
  mainAxisAlignment: MainAxisAlignment.spaceBetween,
  crossAxisAlignment: CrossAxisAlignment.center,