Don't use pub directly

[MarcelGarus]

In Flutter's bin directory, there are only flutter and dart executables, no pub executable. So in order to use screenshots, I can't rely on the Dart that comes pre-bundled with Flutter. Instead, I have to install Dart separately.

It would be great if screenshots could use dart pub instead of relying on pub directly.

[MarcelGarus]

I fixed this by adding C:\src\flutter\bin\cache\dart-sdk\bin in my path.