Running in NW.js enviroment failed

[commyfriend]

When dart-sass is run in the nw.js environment, it failed, and it looks like we need to make a special check for nw.js, just like electron

https://github.com/mbullington/node_preamble.dart/blob/master/lib/preamble.js#L50

process.versions.hasOwnProperty('nw')

[corwin-of-amber]

I have been having this issue for a long time and I'm using this horrible monkey patch:

if (typeof process !== 'undefined' && process.versions['nw'])
    process.versions['electron'] = 'FAKE (hack for node_preamble.dart)';

It should be a simple fix of adding nw to the condition. Are PRs considered here?