fix(forms): introduce checkbox required validator

Closes #11459
Closes #13364

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

@@ -64,6 +64,13 @@ export class Validators {
     return isEmptyInputValue(control.value) ? {'required': true} : null;
   }
 
+  /**
+   * Validator that requires control value to be true.
+   */
+  static requiredTrue(control: AbstractControl): {[key: string]: boolean} {
+    return control.value === true ? null : {'required': true};
+  }
+
   /**
    * Validator that requires controls to have a value of a minimum length.
    */