describe('static', function() {
    describe('method `getAll`', function() {
        ...
        beforeEach(function() {
            $httpBackend.whenPOST(url, getPanelsForCategory)
                .respond(mockPanelsForCategory);
            $httpBackend.whenPOST(url, getPanelDetails)
                .respond(mockPanel);
        });

        it('should work... Please...', function() {
            $httpBackend.expectPOST(url, getPanelsForCategory);

            Panel.getAll('category', 1).then(function(panels) {
                // Here everything falls down.
            });

            $httpBackend.flush();
        });
    });
});

Panel.get = function(id) {
        return API.getPanel(id).then(function(panel) {
            return new Panel(panel);
        });
    };

    Panel.getAll = function(id) {
        var handleResponse = function(data) {
            var promise = $q.all(
                data.map(function(panelRaw) {
                    var panel = Panel.get(panelRaw.PANEL_ID);
                    return panel;
                })
            );

            return promise;
        };

        return API.getPanelsForCategory(id).then(handleResponse);
    };