Dart: How to remove leading spaces in a multiline string (using splitMapJoin)

As far as I know, there’s no built-in way to remove spaces from a multiline string in Dart, but you can write your own function to do this. For example, I just dug into the splitMapJoin method of the Dart String class, and wrote a stripMargin function like this:

String stripMargin(String s) {
    return s.splitMapJoin(
        RegExp(r'^', multiLine: true),
        onMatch: (_) => '\n',
        onNonMatch: (n) => n.trim(),