protractor: `Keys` is not defined when using `keyDown` action

When trying to create a test which checks the result of an option + click() event in my app, Keys is not defined.

This is the test being run (apologies if the JS looks malformed, it’s converted from CoffeeScript):

var driver = new webdriver.Builder()
    .usingServer('http://localhost:4444/wd/hub')
    .withCapabilities(webdriver.Capabilities.chrome()).build()

driver.manage().timeouts().setScriptTimeout(30000)
var ptor = protractor.wrapDriver(driver)

it('opens new tab with OPTION+click', function() {
    ptor.keyDown(Keys.ALT)
        .then(function() {
            ptor.findElement(protractor.By.repeater('item in ses.items').row(4).column("{{item.subject}}")).click();
            ptor.keyUp(Keys.ALT);
        })

    ptor.findElements(protractor.By.repeater('item in ses.navItems'));
        .then(function(tabs) {
            expect(tabs.length).toBe(1);
        });
});

I’ve also tried the following variations for Keys.ALT: ptor.driver.Keys.ALT, ptor.driver.Key.ALT, and Key.ALT

It’s entirely possible I’m not writing this test correctly. If that’s the case I’d appreciate any pointers in testing clicks with key modifiers.