$httpBackend expectGET not returning expected data

I will just start out saying that $httpBackend and spyOn are pretty new to me, and I am struggling to get a clear picture of what is happening. Basically, I'm trying to do an $httpBackend.expectGET('/myroute/').respond(...) and then use spyOn, but I keep getting

Expected [  ] to equal Object({ success: true, message: 'Yeah!' })

Here's the code I'm trying to test:

angular.module('TestModule')
       .service('TestService',TestService);

TestService.$inject = ['$http'];
function TestService($http){
    var service = {};
    service.DoIt = DoIt;
    return service;

    function DoIt() {
        return $http.get('/myroute/')
            .then(handleSuccess,handleError('Error happened!'));
    }

    function handleSuccess(res) {
        return res.data;
    }

    function handleError(error) {
        return function(){
            return {success: false, message: error}
        };
    }
}

Here's my Karma-Jasmine test:

describe('Test Module',function(){
    beforeEach(module('TestModule'));

    describe('TestService Tests',function(){
        var service,$httpBackend;
        beforeEach(inject([
             '$injector',
             function($injector){
                 $httpBackend = $injector.get('$httpBackend');
                 service = $injector.get('TestService');
             }
        ]));

        afterEach(function(){
            $httpBackend.verifyNoOutstandingExpectation();
            $httpBackend.verifyNoOutstandingRequest();
        });

        it('should DoIt',function(){
            var mockResponse = {
                data : {success:true, message:"Yeah!"}
            };

            $httpBackend.expectGET('/myroute')
                        .respond(Promise.resolve(mockResponse));

            spyOn(service,'DoIt').and.callThrough();

            service.DoIt().then(function(data){
                expect(service.DoIt).toHaveBeenCalled();
                expect(data).toEqual(mockResponse.data); <-- ERROR OCCURS HERE -->
            });

            $httpBackend.flush();
        });

    });
});

Further information:

Using the example I found here: http://www.bradoncode.com/blog/2015/06/26/unit-testing-http-ngmock-fundamentals/, I tried this in my test code:

it('should DoIt',inject(function($http){
    var promise = $http.get('/myroute').then(handleSuccess,handleError('bad stuff happened'));
    promise.then(function(data){
        console.log('Data: ' + JSON.stringify(data,null,3));
    });

    $httpBackend.expect('GET','/myroute').respond(200,{data:'Yeah!'});
    $httpBackend.flush();
}));

function handleSuccess(response) {return response.data;}
function handleError(error){
    return function(){
        return {success:false,message:error};
    };
}

This gives me the response that I expect: 'Data: {"data": "yeah"}'

My updated question: I thought service.DoIt() was doing the same thing as the promise. What is going on here? Am I not injecting my service? I do not understand how Karma expects me to stage this test. Any help, please!

If I understand your intention with the test correctly, I believe this is what you need:

describe('Test Module', function() {

    var $httpBackend, service;

    beforeEach(module('TestService'));

    beforeEach(inject(function(_$httpBackend_, _TestService_) {
        $httpBackend = _$httpBackend_;
        service = _TestService_;
    }));

    afterEach(function() {
        $httpBackend.verifyNoOutstandingExpectation();
        $httpBackend.verifyNoOutstandingRequest();
    });

    it('should DoIt', function() {
        var mockResponse = {
            data: { success: true, message: "Yeah!" }
        };

        $httpBackend.expectGET('/myroute/')
            .respond(mockResponse);

        service.DoIt().then(function(data) {
            expect(data).toEqual(mockResponse);
        });

        $httpBackend.flush();
    });

});