INJECTOR
一个 InjectionToken，用于获取当前 Injector 的 createInjector() 式的注入器。
An InjectionToken that gets the current Injector for createInjector()-style injectors.
const INJECTOR: InjectionToken<Injector>;
说明
请求此令牌而不是 Injector 可使 StaticInjector 能在项目中摇树优化掉。
Requesting this token instead of Injector allows StaticInjector to be tree-shaken from a project.
