Breakpoints set in a Dart web app with vscode are not being before the app runs

Repro:
1. Open the example directory (https://github.com/dart-lang/webdev/tree/main/example) in vscode.
2. Set a breakpoint in main.dart on line 13. https://github.com/dart-lang/webdev/blob/5b7299f5b85706536aeac1c9f470b869ddb39fef/example/web/main.dart#L13
3. Click the debug link vscode inserts above the main method.

You should see the app build and start running, chrome pops up and there will be prints appearing in the vscode debug console.

```
Initial Print :)
Counter is now: 1
Counter is now: 2
Counter is now: 3
Counter is now: 4
...
```

4. Now add a breakpoint on line 23. https://github.com/dart-lang/webdev/blob/5b7299f5b85706536aeac1c9f470b869ddb39fef/example/web/main.dart#L23

You should see execution will pause.

5. Click the "Restart" button in the vscode debugging UI to hot restart the app.

You should see that the breakpoint on line 13 is skipped again but the breakpoint on line 23 still works.

Reproduce the behavior from the example directory using example/web/main.dart, setting breakpoints on lines 13 and 23 and restarting through the VS Code debugging UI. Start by tracing how the Dart web debug session handles breakpoints across hot restart. Done means the breakpoint on line 13 is hit after restart while the breakpoint on line 23 continues to work.

Written by the indexing model from the issue text.