// at the top of the test spec:
var fs = require('fs');

// ... other code

// abstract writing screen shot to a file
function writeScreenShot(data, filename) {
    var stream = fs.createWriteStream(filename);

    stream.write(new Buffer(data, 'base64'));
    stream.end();
}

// ...

// within a test:
browser.takeScreenshot().then(function (png) {
    writeScreenShot(png, 'exception.png');
});