add unit test for main frontend application

frontend/src/app.spec.js

@@ -0,0 +1,52 @@
+import { App } from "./app";
+
+const TEST_UTILS = [
+  { name: 'util1' },
+  { name: 'util2' },
+];
+
+describe('App', () => {
+  let app;
+
+  beforeEach(() => {
+    app = new App();
+  });
+
+  it('should create', () => {
+    expect(app).toBeTruthy();
+  });
+
+  describe('provides services', () => {
+    it('HttpClient as httpClient', () => {
+      expect(app.httpClient).toBeTruthy();
+    });
+
+    it('HtmlHelpers as htmlHelpers', () => {
+      expect(app.htmlHelpers).toBeTruthy();
+    });
+
+    it('I18n as i18n', () => {
+      expect(app.i18n).toBeTruthy();
+    });
+
+    it('UtilRegistry as utilRegistry', () => {
+      expect(app.utilRegistry).toBeTruthy();
+    });
+  });
+
+  describe('registerUtilities()', () => {
+    it('should register the given utilities', () => {
+      spyOn(app.utilRegistry, 'register');
+      app.registerUtilities(TEST_UTILS);
+      expect(app.utilRegistry.register.calls.count()).toBe(TEST_UTILS.length);
+      expect(app.utilRegistry.register.calls.argsFor(0)).toEqual([TEST_UTILS[0]]);
+      expect(app.utilRegistry.register.calls.argsFor(1)).toEqual([TEST_UTILS[1]]);
+    });
+
+    it('should throw an error if not passed an array of utilities', () => {
+      expect(() => {
+        app.registerUtilities({});
+      }).toThrow();
+    });
+  });
+});