AngularJS ControllerAs syntax and controller injected variables

I'm writing a directive and trying to stick to the John Papa style guide. So I've also decided to jump on the ControllerAs syntax wagon and I've got a tiny directive like below:

(function() {
    angular
        .module('htApp')
        .directive('newsletterSignup', newsletter_signup);

    function newsletter_signup($http) {
        var directive = {
            templateUrl: '../whatever.tpl.html',
            restrict: 'EA',
            controller : controller,
            controllerAs: 'vm',
            bindToController: true
        };

        return directive;

        controller.$inject = ['$http'];

        function controller($http) {
            var vm = this;
            // $http is here ... all is good, but I don't need it

            function doSubmit(form) {
                // I need $http here, but it is null
                debugger;
            };

            vm.doSubmit = doSubmit;
        }
    }
})();

This is a newsletter signup service. I'm going to have to do an HTTP request, therefore I'm injecting it into the controller. All is fine - but calling the vm.doSubmit(--formname-here--) function from the template results in me not being able to find the $http service.

So my question: how can I access the injected $http from the doSubmit() function?

EDIT

I'll include the view code - but no worries - the plumbing works:

<button class="btn btn-yellow" ng-click="vm.doSubmit(newsletterform)" translate>
    footer.ok_button_text
</button>

EDIT 2

As it turns out, @Tek was right - the code works. I think the reason I didn't see it was because (I think) the JS runtime in Chrome optimizes the $http away when it knows it's not going to be called.

This code works fine. I think this is because the runtime aniticipated the usage of $http in the console.log() function call. However - if I remove that line I get this ( which was why I had this problem in the first place ):

Notice that I commented out the console.log - thus the doSubmit() call never uses $http. Now - when I call $http in the console - it's not defined.