错误消息：
ERROR NullInjectorError: R3InjectorError(AppModule)[MyService -> MyNewService -> MyNewService -> MyNewService]:
NullInjectorError: No provider for MyNewService!
at NullInjector.get (http://localhost:4200/vendor.js:62758:27)
at R3Injector.get (http://localhost:4200/vendor.js:76765:33)
at R3Injector.get (http://localhost:4200/vendor.js:76765:33)
at R3Injector.get (http://localhost:4200/vendor.js:76765:33)
at injectInjectorOnly (http://localhost:4200/vendor.js:62613:33)
at ɵɵinject (http://localhost:4200/vendor.js:62623:57)
at Object.factory (http://localhost:4200/vendor.js:77142:19)
at R3Injector.hydrate (http://localhost:4200/vendor.js:77003:63)
at R3Injector.get (http://localhost:4200/vendor.js:76753:33)
at NgModuleRef$1.get (http://localhost:4200/vendor.js:94067:33)
查看我的依赖注入相关的代码：
发现上图的MyNewService在app.module.ts里缺少了providers定义。
将这个缺失的定义加入之后，问题消失：
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