Test Service defined for testing REST calls with HTTPClient library

test.service.ts

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { Observable } from 'rxjs/Observable';

@Injectable()
export class TestService {
  private testUrl = '/some/test/url';

  constructor(private http: HttpClient) { }

  public testCall(): Observable<string> {
    return this.http.get(this.testUrl);
  }
}