complete_example_test.dart

import 'package:test/test.dart';
import 'package:mocktail/mocktail.dart';

class Demo {
  void doSomething(int i) {
    i + i;
  }
}

class DemoDouble extends Mock implements Demo {}

class UserOfDemo {
  final Demo _demo;

  UserOfDemo(this._demo);

  void methodThatDependsOnDemo() {
    _demo.doSomething(1);
  }
}

void main() {
  test('it calls doSomething()', () {
    final _demo = DemoDouble();
    final sut = UserOfDemo(_demo);
    sut.methodThatDependsOnDemo();
    verify(_demo).called(#doSomething).withArgs(positional: [1]);
  });
}

The following should work:

import 'package:test/test.dart';
import 'package:mocktail/mocktail.dart';

class Demo {
  void doSomething(int i) {
    i + i;
  }
}

class DemoDouble extends Mock implements Demo {}

class UserOfDemo {
  final Demo _demo;

  UserOfDemo(this._demo);

  void methodThatDependsOnDemo() {
    _demo.doSomething(1);
  }
}

void main() {
  test('it calls doSomething()', () {
    final _demo = DemoDouble();
    when(_demo).calls(#doSomething).thenReturn(null);
    final sut = UserOfDemo(_demo);
    sut.methodThatDependsOnDemo();
    verify(_demo).called(#doSomething).withArgs(positional: [1]);
  });
}