devtools_extensions: asyncEval fails on web targets

This issue is for devtools_extensions.

`asyncEval` is not working for debug services running for web targets. Other targets (tested on MacOS) work fine.

Whenever I call `asyncEval` on a web target I get the following error:

```
ChromeProxyService: Failed to evaluate expression 'postEvent': InternalError: frontend server failed to compile '() { return
postEvent; }'.
```

A simple eval like this already throws the error:

```dart
final eval = EvalOnDartLibrary(
'dart:core',
serviceManager.service!,
serviceManager: serviceManager,
);

await eval.asyncEval('Future.value(42)', isAlive: isAlive)
```

@kenzieschmoll I encountered this error while working on the [shared_preferences_tools issue.](https://github.com/flutter/flutter/issues/145433#issuecomment-2112842958)