属性'list'在离子和属性'Firebasedatabse'中不存在'List'在离子型'Firebasedatabse'中不存在

Question

在我的IDE中list和object加下划线。你能告诉我为什么吗？

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { AngularFireDatabaseModule, AngularFireDatabase, AngularFireList, AngularFireObject  } from 'angularfire2/database';
import { Observable } from 'rxjs/Observable';