The command "pub" is either misspelled or could not be found

We migrated from flutter 1x to flutter 2.2.0.

**Environment**

* Windows 10
* flutter 2.2.0
* dart 2.13.0 (installed by flutter)
* protoc 3.17.0
* protoc_plugin 20.0.0 (installed with `flutter pub global activate protoc_plugin 20.0.0`)

**Issue**

Running `protoc --dart_out=grpc [...]` generates the fowllowing (german) error:

```
Der Befehl "pub" ist entweder falsch geschrieben oder konnte nicht gefunden werden.
--dart_out: protoc-gen-dart: Plugin failed with status code 1.
```

(Probably `The command "pub" is either misspelled or could not be found`)

The `protoc-gen-dart.bat` contains the following line:

```
pub global run protoc_plugin:protoc_plugin %*
```

The folder `flutter\bin` only contains the following files:

* flutter
* flutter.bat
* dart
* dart,bat

The folger `flutter\bin\cache\dart-sdk\bin` contains:

* dart.exe
* pub.bat

The command `pub` does not exists in any dart folder.

**Workaround**

I added `dart` in front of `pub` within the `protoc-gen-dart.bat`:

```
dart pub global run protoc_plugin:protoc_plugin %*
```

Now, `protoc --dart_out=grpc [...]` generates the dart files.

**Conclusion**

The `protoc-gen-dart.bat` is not generated correct.

Start by locating the generator or template that produces protoc-gen-dart.bat, then inspect how the Windows launcher invokes protoc_plugin. Reproduce the failure with the listed Flutter, Dart, protoc, and protoc_plugin versions, and confirm that protoc --dart_out=grpc succeeds without the manual workaround.

Written by the indexing model from the issue text.