Cannot perform http mock while integration testing flutter

Question

import 'package:http/http.dart' as http;
import 'package:myapp/main.dart' as app;

void main() {
  IntegrationTestWidgetsFlutterBinding.ensureInitialized();
  group(
  'todo API test',() {testWidgets(
        'verify todo app functionality with data retrieval failure',
        (WidgetTester tester) async {
          app.main();
          await tester.pumpAndSettle();
          await app.main();
          await tester.pumpAndSettle();
          await Future.delayed(const Duration(seconds: 2));
          await tester.tap(find.byKey(const Key('carKey')));
          await Future.delayed(const Duration(seconds: 2));
          await tester.enterText(find.byType(TextFormField).at(0), 'username');
          await Future.delayed(const Duration(seconds: 2));
          await tester.enterText(find.byType(TextFormField).at(1), 'password');
          await Future.delayed(const Duration(seconds: 2));
          await tester.tap(find.byType(ElevatedButton));
          await Future.delayed(const Duration(seconds: 2));
          await tester.pumpAndSettle();

          await Future.delayed(const Duration(seconds: 2));
          // Add a todo
          await tester.enterText(find.byType(TextField), 'New Todo');
          await tester.tap(find.byType(ElevatedButton));
          await tester.pumpAndSettle();
          expect(find.text('New Todo'), findsOneWidget);
          await Future.delayed(const Duration(seconds: 2));
          await tester.tap(find.byIcon(Icons.delete).first);
          await tester.pumpAndSettle();
          await Future.delayed(const Duration(seconds: 2));
          expect(find.text('New Todo'), findsNothing);
          await Future.delayed(const Duration(seconds: 2));
          await tester.tap(find.byKey(const Key('refreshButton')));
          await tester.pumpAndSettle();
          await Future.delayed(const Duration(seconds: 2));
          // Verify that the todo list is updated after fetching again
          expect(find.text('New Todo'), findsNothing);
          await Future.delayed(const Duration(seconds: 2));
          // Mock a failure response for data retrieval
          final mockClient = MockClient((request) async {
            return http.Response(
                'Error', 500); // Return 500 Internal Server Error
          });

          // Replace the HTTP client with the mock client::Error here
          http.Client = mockClient;

          // Refresh todo list with the mocked HTTP client
          final response = await http
              .get(Uri.parse('https://jsonplaceholder.typicode.com/todos'));
          expect(response.statusCode, 500);
          await Future.delayed(const Duration(seconds: 2));
          log("Failed to fetch data");
        },
      );
    },
  );
}

Cannot use the line http.Client = mockClient; Am receiving the error: The name 'Client' is being referenced through the prefix 'http', but it isn't defined in any of the libraries imported using that prefix. Try correcting the prefix or importing the library that defines 'Client'.