fragment generation tests

[klavs]

• add __typename field to object and interface definitions
• add some fragment-related tests

[klavs]

@smkhalsa, here's the API I originally imagined. What do you think? I like this approach because it lets user avoid using generated classes directly.

[smkhalsa]

One minor note: if we did this, we'd probably want to prefix the asSubclass() methods with $ to avoid collisions with data fields.

What I like about using the generated subclasses directly is that if (class is classAsSubclass) feels intuitive and doesn't require any additional typecasting.

That said, if we did go this route, it would feel more intuitive to me to also have a $isSubclass property rather than checking for null with the $asSubclass() method.

if (shape.$isSquare) {
  final square = shape.$asSquare();
  return "$type(area: $area, sideLength: ${square.sideLength})";
}

if (shape.$isRectangle) {
  final rectangle = shape.$asRectangle();
  return "$type(area: $area, sideLengthA: ${rectangle.sideLengthA}, sideLengthB: ${rectangle.sideLengthB})";
}