Support for Windows

[exorsctum]

Recently I checked this nvim config for Dart and want to use, but the installer "setup.py" is not working in Windows. So, adding support for Windows will be great. :D

[AlixShahid]

i don't have windows-OS for testing. it would be great if someone make PR.

[ryannewcomer]

hi, i am running on window, and it won't word:

[AlixShahid]

@ryannewcomer sorry but as i mentioned previously, i don't have window OS. waiting for someone to make a PR