Request FxA login on Firefox OS 2.0 and later (bug 1033696)

I really have no idea how you'd write automated tests for this.
instead of polluting the global navigator (which might affect subsequent unit tests), make the utility function take an option like this:

    function supportsNativeFxA(opt) {
        opt = opt || {navigator: navigator};
        return opt.navigator.mozId ...
    }

Now in your tests you can pass in a stub:

    utils.supportsNativeFxA({navigator: {...}})

In real code, calling ``utils.supportsNativeFxA()`` will do the right thing. Even though it's not executed exactly like that in the test, you've minimized the variation to one line of code, the ``opt = opt || ...`` line so test coverage is still high.