Function validator

packages/forms/test/validators_spec.ts:26–32

(key: string, error: any)

(function () {
  function validator(key: string, error: any): ValidatorFn {
    return (c: AbstractControl) => {
      const r: ValidationErrors = {};
      r[key] = error;
      return r;
    };
  }

  class AsyncValidatorDirective implements AsyncValidator {
    constructor(