如何在聚合物Dart中执行自定义验证

import 'package:polymer/polymer.dart';
import 'package:web_components/web_components.dart';

@CustomTag('my-custom-validator')
class MyCustomValidator extends PolymerElement with Validator {
  @override
  bool get isValid => _validate(value);

  String get value => (shadowRoot.querySelector('#inputField') as InputElement).value;

  bool _validate(String value) {
    if (value == null || value.isEmpty) return false;
    try {
      int number = int.parse(value);
      return number > 0;
    } catch (e) {
      return false;
    }
  }
}

<my-custom-validator>
  <input id="inputField" type="text">
</my-custom-validator>