What is the dart function type syntax for variable declaration?

You can use typedef :

typedef Comparison<T> = int Function(T a, T b);

class A<T> {
  Comparison<T> compare;
}

main() {
  A a = new A<int>();
  a.compare = (int a, int b) => a.compareTo(b);
  print(a.compare(1, 2));
}