Take this minimal Point class:

class Point {
  num x;
  num y;
  Point(this.x, this.y);

  num distanceTo(Point other) {
    ...
  }
}

As you write tests for Point, you will probably want to set up one or more point objects that you can access in each test. Something like:

void main() {
  group("Point", (){
    setUp((){
      Point p1 = new Point(3, 4);
      Point p2 = new Point(3, 5);
    });
    test("distanceTo()", (){
      expect(p1.distanceTo(p2), equals(...));
    });
  });
}

Do this and the Editor starts complaining that it cannot make sense of p1 or p2. Why? Remember, setUp() simply calls the function passed to it before each test() and our code defining p1 and p2 will therefore run every time. But (and this seems like a Captain Obvious mement) because p1 and p2 are local to function called by setUp(), they cannot be accessed from outside that function. But it takes only very small changes to take care of the access problem:

void main() {
  group("Point", (){
    Point p1;
    Point p2;
    setUp((){
      p1 = new Point(3, 4);
      p2 = new Point(3, 5);
    });
    test("distanceTo()", (){
      expect(p1.distanceTo(p2), equals(34));
    });
  });
});

Now, the function called by setUp() assigns a value to the already existing p1 and p2.