title: Difference between NG_VALIDATORS and validators (Class)
body: <p>I'm confused on what's the usage of NG_VALIDATORS, I know it is a provider token. But what's its use? How is it any different form validators class?</p>
  
  <p><a href="https://github.com/angular/angular/blob/4.3.3/packages/forms/src/validators.ts" rel="noreferrer">https://github.com/angular/angular/blob/4.3.3/packages/forms/src/validators.ts</a></p>