(Somewhat similar to #80048, but a distinct feature.)

A common NNBD pattern for data objects in Dart is to have optional collection members be non-nullable with default empty values, to avoid having two different ways of expressing "nothing" (null vs. empty). So an object where no arguments are required would look like:

class Foo {
  Foo({
    this.aString,
    this.aDouble,
    this.aList = const List<String>[],
    this.aMap = const Map<String, int>{},
  });

  String? aString;
  double? aDouble;
  List<String> aList;
  Map<String, int> aMap;
}

Since Pigeon doesn't allow default parameter values, we currently have to make the collections nullable which makes them error-prone on the native side (we've had real bugs due to native code checking for only one type of "empty"), or we have to make the parameters required which makes them more annoying to create on the Dart side.