feat(forms): expose ValidatorFn and AsyncValidatorFn

Closes #8834

Author: kara

modules/@angular/forms/src/forms.ts

@@ -38,8 +38,9 @@ export {FormControlName} from './directives/reactive_directives/form_control_nam
 export {FormGroupDirective} from './directives/reactive_directives/form_group_directive';
 export {FormGroupName} from './directives/reactive_directives/form_group_name';
 export {NgSelectOption, SelectControlValueAccessor} from './directives/select_control_value_accessor';
-export {MaxLengthValidator, MinLengthValidator, PatternValidator, RequiredValidator, Validator} from './directives/validators';
+export {AsyncValidatorFn, MaxLengthValidator, MinLengthValidator, PatternValidator, RequiredValidator, Validator, ValidatorFn} from './directives/validators';
 export {FormBuilder} from './form_builder';
 export {AbstractControl, FormArray, FormControl, FormGroup} from './model';
 export {NG_ASYNC_VALIDATORS, NG_VALIDATORS, Validators} from './validators';
+
 export * from './form_providers';

tools/public_api_guard/forms/index.d.ts

@@ -58,6 +58,10 @@ export declare abstract class AbstractControlDirective {
     valueChanges: Observable<any>;
 }
 
+export interface AsyncValidatorFn {
+    (c: AbstractControl): any;
+}
+
 export declare class CheckboxControlValueAccessor implements ControlValueAccessor {
     onChange: (_: any) => void;
     onTouched: () => void;
@@ -327,6 +331,12 @@ export interface Validator {
     };
 }
 
+export interface ValidatorFn {
+    (c: AbstractControl): {
+        [key: string]: any;
+    };
+}
+
 export declare class Validators {
     static compose(validators: ValidatorFn[]): ValidatorFn;
     static composeAsync(validators: AsyncValidatorFn[]): AsyncValidatorFn;