If the dependency is added to a module, it gets registered on the root injector.
如果依赖添加到module上，那么，这些服务会注册到root injector。
angular每个组件都有对应的injector,然后组成了一个injector tree
详细介绍：https://blog.angularindepth.com/angular-dependency-injection-and-tree-shakeable-tokens-4588a8f70d5d