How to perform async validation using reactive/model-driven forms in Angular 2

I have an email input and I want to create a validator to check, through an API, if the entered email it's already in the database.

So, I have:

A validator directive

import { Directive, forwardRef } from '@angular/core';
import { Http } from '@angular/http';
import { NG_ASYNC_VALIDATORS, FormControl } from '@angular/forms';

export function validateExistentEmailFactory(http: Http) {
  return (c: FormControl) => {
    return new Promise((resolve, reject) => {
      let observable: any = http.get('/api/check?email=' + c.value).map((response) => {
        return response.json().account_exists;
      });

      observable.subscribe(exist => {
        if (exist) {
          resolve({ existentEmail: true });
        } else {
          resolve(null);
        }
      });
   });
  };
}

@Directive({
  selector: '[validateExistentEmail][ngModel],[validateExistentEmail][formControl]',
  providers: [
    Http,
    { provide: NG_ASYNC_VALIDATORS, useExisting: forwardRef(() => ExistentEmailValidator), multi: true },
  ],
})
export class ExistentEmailValidator {
  private validator: Function;

  constructor(
    private http: Http
  ) {
    this.validator = validateExistentEmailFactory(http);
  }

  public validate(c: FormControl) {
    return this.validator(c);
  }
}

A component

import { Component } from '@angular/core';
import { FormGroup, FormBuilder, Validators } from '@angular/forms';
import { ExistentEmailValidator } from '../../directives/existent-email-validator';

@Component({
  selector: 'user-account',
  template: require<string>('./user-account.component.html'),
})
export class UserAccountComponent {
  private registrationForm: FormGroup;
  private registrationFormBuilder: FormBuilder;
  private existentEmailValidator: ExistentEmailValidator;

  constructor(
    registrationFormBuilder: FormBuilder,
    existentEmailValidator: ExistentEmailValidator
  ) {
    this.registrationFormBuilder = registrationFormBuilder;
    this.existentEmailValidator = existentEmailValidator;

    this.initRegistrationForm();
  }

  private initRegistrationForm() {
    this.registrationForm = this.registrationFormBuilder.group({
      email: ['', [this.existentEmailValidator]],
    });
  }
}

And a template

<form novalidate [formGroup]="registrationForm">
  <input type="text" [formControl]="registrationForm.controls.email" name="registration_email" />
</form>

A've made other validator this way (without the async part) and works well. I think te problem it's related with the promise. I'm pretty sure the code inside observable.subscribe it's running fine. What am I missing?

I'm using angular v2.1