仅在Angular 2中导入模块一次

Fairly new to angular 2 and still learning. As far as I have read about using Modules in Angular 2 and importing them in the application is that they are shared with their children. In the code below I am trying to share ReactiveFormsModule application wide, but application crashes while compiling. I did accomplish this for my custom modules using the forRoot method, but I can't understand how to do it with the builtin ngModules.Fairly new to angular 2 and still learning. As