What happens when you're missing variance

main.dart

class Foo {}
class Bar extends Foo {}

void main() {
  Map<String, Foo> m = {};
  Map<String, Bar> n = {};
  m = n;
  m["a"] = Foo();
  // Uncaught Error: TypeError: Instance of 'Foo': type 'Foo' is not a subtype of type 'Bar'
}