calculus-widget-test.dart

void main() {
  group('Calculus screen', () {
    ...
    testWidgets('has one operand when operation is SQRT', (WidgetTester tester) async {
      // initiate required state
      CalculatorState state = CalculatorState();
      state.setOperation(Sqrt);

      await tester.pumpWidget(MaterialApp(
        home: ChangeNotifierProvider<CalculatorState>(
          create: (_) => state, // passing state to widgtes
          builder: (context, _) => CalculusScreenContainer(),
        ),
      ));
      expect(find.text('Operand A'), findsOneWidget);
      expect(find.text('Operand B'), findsNothing);
    });
  });
}