如何在函数中设置默认值

编译时常量对于 DateTime.now() .

/// Returns `true` is still going on.
///
/// [date] the date to check.
///   as default value `DateTime.now()` is used 
///   if no value or `null` was passed.
bool isOnGoing([DateTime date]) {
    date ??= DateTime.now();
    ...
}