Angular 2提供的参数与调用目标的任何签名都不匹配(spec.ts)

constructor(private _elRef: ElementRef) {}

当我构建(npm start)时,我收到此错误

...angular2/tmp/broccoli_type_script_compiler-input_base_path-wKrIZXNv.tmp/0/src/app/notifications/notifications.component.spec.ts (10,21): Supplied parameters do not match any signature of call target

angular cli生成的文件notifications.component.spec.ts是这样的

import { By }           from '@angular/platform-browser';
import { DebugElement } from '@angular/core';
import { addProviders,async,inject } from '@angular/core/testing';
import { NotificationsComponent } from './notifications.component';

describe('Component: Notifications',() => {
 it('should create an instance',() => {
 let component = new NotificationsComponent();
 expect(component).toBeTruthy();
 });
});

但是,如果我在没有构造函数参数的情况下构建,一切正常.如果我在构建后添加此参数,一切都正常.

我错过了什么？