This brings the Windows script up to par with the Linux/Mac script.

`flutter doctor` works.

There are two scripts (`bin/flutter` and `bin/internal/update_dart-sdk`), which are platform dependent (bash script on Linux/Max, PowerShell script on Windows) and we need to keep an eye on making sure that their logic stays in sync. Both scripts are rather simple and I am not expecting many changes to them. I also made sure that both versions follow the same structure to make it easier to keep them consistent.

Required for https://github.com/flutter/flutter/issues/138