How do I mock path.resolve() when it's private?

I have a private function that I am trying to mock path.resolve() using Mocha, Chai, and Sinon.

Right now, I get TypeError: Arguments to path.resolve must be strings for var projectDir = path.resolve(__dirname + "../../../");.. I am not sure how to go about this because it's private I can't mock it...and it doesn't take argument functions so I can't feed it anything.

Any suggestions?

Node script:

function constructDestwCallBack(absSrcFile, callback) {
    console.log(path.resolve(__dirname + "../../../"))
    var projectDir = path.resolve(__dirname + "../../../");
    ...

function foo(callback) {
    var destinationFile;
    errorCheckArg(arguments);
    return through2.obj(function(file, enc, next) {
        destinationFile = constructDestwCallBack(absSrcFile, callback);

---

Test:

describe('gulp-foo', function() {
    var fakeFile, pspy;

    beforeEach(function() {
        mock({
            '/apple/pear/foo.less': mock.file({
            content: 'nothing',
            mtime: new Date(Date.now())
            })
        });
        fakeFile = new File({
            contents: new Buffer('/apple/pear/foo.less')
        });

    });
    afterEach(mock.restore);


    describe('get files', function() {

        it('should do something', function(done) {
            var path = { resolve: function() { return "ssss"} };
            sinon.spy(path, "resolve");


            var bar = function(dest) { return dest};
            stream = foo(bar);
            done();
        });
    });