我想为服务编写单元测试，但出现错误提示“ TypeError：_this.handler.handle不是函数”

Question

我有一个服务，该服务有一个加载功能，我想测试该功能，并得到一个错误。 “ TypeError：_this.handler.handle不是函数”

这是我的服务，该服务正常运行。

export class AppConfigService {

    static settings: IAppConfig;
    constructor(private http: HttpClient) {}

    load() {
        const jsonFile = `assets/config/config.${environment.name}.json`;
        return new Promise<void>((resolve, reject) => {
            this.http.get(jsonFile).toPromise().then((response: IAppConfig) => {
                AppConfigService.settings = response as IAppConfig;
                resolve();
            }).catch((response: any) => {
                console.log(response);
                reject(`Could not load file '${jsonFile}': ${JSON.stringify(response)}`);
            });
        });
    }
}

这是我的考验

describe('AppConfigService', () => {
    let service: AppConfigService;
    beforeEach(() => {
        TestBed.configureTestingModule({
            providers: [
                AppConfigService,
                HttpClient,
                HttpHandler,
                HttpBackend,
            ]});
        service = TestBed.get(AppConfigService);
    });

    it('should be created', () => {
        expect(service).toBeTruthy();
    });
    it('should ran load well', () => {
        service.load();
    });
});

然后我收到此错误：

TypeError: _this.handler.handle is not a function
TypeError: _this.handler.handle is not a function
    at MergeMapSubscriber.project (http://localhost:9876/_karma_webpack_/webpack:/node_modules/@angular/common/fesm5/http.js:1046:62)
    at MergeMapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/mergeMap.js.MergeMapSubscriber._tryNext (http://localhost:9876/_karma_webpack_/webpack:/node_modules/rxjs/_esm5/internal/operators/mergeMap.js:61:1)
    at MergeMapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/mergeMap.js.MergeMapSubscriber._next (http://localhost:9876/_karma_webpack_/webpack:/node_modules/rxjs/_esm5/internal/operators/mergeMap.js:51:1)
    at MergeMapSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next (http://localhost:9876/_karma_webpack_/webpack:/node_modules/rxjs/_esm5/internal/Subscriber.js:54:1)
    at Observable._subscribe (http://localhost:9876/_karma_webpack_/webpack:/node_modules/rxjs/_esm5/internal/observable/scalar.js:5:1)
    at Observable.push../node_modules/rxjs/_esm5/internal/Observable.js.Observable._trySubscribe (http://localhost:9876/_karma_webpack_/webpack:/node_modules/rxjs/_esm5/internal/Observable.js:43:1)
    at Observable.push../node_modules/rxjs/_esm5/internal/Observable.js.Observable.subscribe (http://localhost:9876/_karma_webpack_/webpack:/node_modules/rxjs/_esm5/internal/Observable.js:29:1)
    at MergeMapOperator.push../node_modules/rxjs/_esm5/internal/operators/mergeMap.js.MergeMapOperator.call (http://localhost:9876/_karma_webpack_/webpack:/node_modules/rxjs/_esm5/internal/operators/mergeMap.js:29:1)
    at Observable.push../node_modules/rxjs/_esm5/internal/Observable.js.Observable.subscribe (http://localhost:9876/_karma_webpack_/webpack:/node_modules/rxjs/_esm5/internal/Observable.js:24:1)
    at FilterOperator.push../node_modules/rxjs/_esm5/internal/operators/filter.js.FilterOperator.call (http://localhost:9876/_karma_webpack_/webpack:/node_modules/rxjs/_esm5/internal/operators/filter.js:15:1)

我认为，在测试代码中我有错误的服务注入，但是我不知道如何解决这个问题。