isn't path.basename(curSource) === file?

root is not used

Let's do the path-joining magic in the very beginning so that it's easier to follow what's going on.

const from = path.join(source, file);
const to = path.join(target, file);
...

let's check for file explicitly. It would not be nice to write over a symlink.

I don't know what you mean by this one.

I mean:

 if (fs.lstatSync(curSource).isDirectory()) {
  ...
} else if (fs.lstatSync(curSource).isFile()) {
  ...
}

doesn't copying everything into a node6/ work? this would spare you from this replace.

I.e.:

removeSync('node6');
copyFolder('lib', 'node6/lib');
copyFolder('test', 'node6/test');
copyFile('index.js', 'node6/index.js');

It gets really tricky because we load some utils from lib, and some utils are hard to convert. There are also paths for things like package.json and .local-chromium which get confusing.