Some variables that are not expandable appear to be expandable

e.g., when inspecting the following:

```
final name = 'Prismo';
```

The little `>` makes it seem the variable is expandable:

Here is where we determine that: https://github.com/flutter/devtools/blob/0f72e7d300aa1d64f4fcee40ee2785e17cd069e1/packages/devtools_app/lib/src/shared/diagnostics/dart_object_node.dart#L269

Start in packages/devtools_app/lib/src/shared/diagnostics/dart_object_node.dart around line 269 and reproduce the inspector behavior with the provided final name example. Trace how the expandable indicator is selected for Dart object nodes. Done means non-expandable variables no longer show the indicator while genuinely expandable variables still do.

Vom Indexierungsmodell aus dem Issue-Text verfasst.