Repository: flutter
Issue: Give better error message when main function is missing
Labels: tool,c: proposal,a: error message,has reproducible steps,P2,found in release: 3.3,found in release: 3.7,team-tool,triaged-tool

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## Use case
When the main function is missing from main.dart, it throws some errors from the engine:
```
[VERBOSE-2:dart_isolate.cc(144)] Could not prepare isolate.
[VERBOSE-2:runtime_controller.cc(396)] Could not create root isolate.
[VERBOSE-2:shell.cc(606)] Could not launch engine with configuration.
```
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<details>
<summary>Code sample</summary>

```dart
import 'package:flutter/material.dart';

// void main() {
//   runApp(const MyApp());
// }

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headlineMedium,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}

```

</details>

## Proposal

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I believe a better solution would be for the tool to throw a more specific message so it's more clear what is wrong. For example, if the lib/main.dart is not found, it throws an error in red saying `Target file "lib/main.dart" not found.`. Could do something similar like `The main() function was not found in "lib/main.dart".`