This should make our scripts compatible with PowerShell 2 or newer. PowerShell 2 was released in October 2009 and shipped with Windows 7 as default. (I suspect the scripts are now also compatible with PowerShell 1, but that's unconfirmed). This fixes#8606.
The PR also introduces better error handling when Flutter fail to download the Dart SDK to fix#8627.
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
