ng-providers.fake-api-service.ts

import { Injectable } from '@angular/core';
import { ApiService } from 'app/api.service';

@Injectable()
export class FakeApiService implements ApiService {
  // Using implements to be sure FakeApiService has the same 
  // methods as original ApiService
  constructor() { }

  serviceCall() {
    console.log('I am FAKE api service instance call!');
  }
}