title: Injecting ngControl in custom validator directive, causes cyclic dependency
body: <p>i'm trying to create custom angular 2 validator directive, which inject NgControl like this :</p>
  
  <pre><code>@Directive({
    selector: '[ngModel][customValidator]',
    providers: [{provide: NG_VALIDATORS, useExisting: CustomValidatorDirective, multi: true}]
  })
  export class CustomValidatorDirective implements Validator {
    private validateFunction: ValidatorFn;
  
    constructor(private control: NgControl) { };
  
  }
  </code></pre>
  
  <p>But i get the following error:</p>
  
  <blockquote>
    <p>Cannot instantiate cyclic dependency! NgControl</p>
  </blockquote>
  
  <p>Does anyone know how i can workarround it, so i can access the ngControl after intialization?</p>