App Size tab doesn't accept AOT snapshots

When I compile a Dart program to an AOT snapshot:

```
$ dart compile aot-snapshot foo.dart
Generated foo.aot
```

I would expect to be able to analyze the snapshot using the App Size Tooling, which claims to support analysis of AOT snapshots. However, the file picker requires a JSON extension and uploading the AOT snapshot does nothing (no error, no change in UI indicating anything happened).

cc @rmacnak-google

Start in the App Size tab's file picker and upload flow, using the dart compile aot-snapshot command to reproduce the issue with foo.aot. Done means an AOT snapshot can be selected and analyzed, with visible UI feedback when the upload succeeds or fails.

Scritto dal modello di indicizzazione a partire dal testo della issue.