can't find the class name "MyApp" in widgets_test.dart file

Initially, in widgets_test.dart, you have this code:

import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

import 'package:app/main.dart';

void main() {
  testWidgets('Counter increments smoke test', (WidgetTester tester) async 
 {
  // Build our app and trigger a frame.
  await tester.pumpWidget(const MyApp());

  // Verify that our counter starts at 0.
  expect(find.text('0'), findsOneWidget);
  expect(find.text('1'), findsNothing);

  // Tap the '+' icon and trigger a frame.
  await tester.tap(find.byIcon(Icons.add));
  await tester.pump();

  // Verify that our counter has incremented.
  expect(find.text('0'), findsNothing);
  expect(find.text('1'), findsOneWidget);
  });
 }

Since you deleted the MyApp class from main.dart file, then you get the error, because the program cant find the class. You can delete the code from widgets_test.dart file, and have just this:

void main() {}

Instead of passing a MaterialApp directly to runApp, wrap it in a MyApp widget and pass MyApp to runApp.

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'My Flutter App',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

In MyApp, there should be a StatelessWidget that contains Material components.

runApp(MyApp();