Dart generics question

main.dart

class Foo<T> extends Object {
  Foo(this._data);
  Foo.construct(this._data);
  
  @override
  String toString() => 'my data is $_data';
  
  final T _data;
}

class NumberFoo extends Foo<num> {
  NumberFoo(num data) : super(data);
}

class Bar<T, F extends Foo<T>> {
  Bar(this._data);
  
  F makeFoo() => F(_data);
  F makeFoo2() => new F(_data);
  F makeFoo3() => F.construct(_data);
  
  final T _data;
}

void main() {
  for (int i = 0; i < 5; i++) {
    Bar<num, NumberFoo> bar;
    print(bar.makeFoo());
  }
}