Coverage resolver fails with a package_config.json on windows

[jonahwilliams]

I have not found the stack/trace error for this yet, but when trying to migrate the flutter_tool to prefer package_config.json I hit this on our CI: flutter/flutter@e8d4e30

Filling as an FYI until I figure out what is going on

[liamappelbe]

Fixed by dart-archive/coverage#371