Taking total control of PaperInput validation

I'm using PaperInput and like the feel. But, is there a way to do the validation using my own logic? For instance, in some cases a pattern match is not enough to determine the error I'd like to display. An example would be I want the PaperInput to specify an item which can only be added once, so the validation would do a lookup in some model map and if input.inputValue is not present it is valid, otherwise invalid.

  <paper-input floatingLabel
               id="alias-input"
               validate="{{aliasIsValid}}"
               type="text"
               error="{{aliasError}}"
               label="Person Alias (eg: King, Eldest Son, Mooch, etc.)"
               required
               ></paper-input>

So, I would like to be able to implement bool aliasIsValid() and set @observable String aliasError when validation is invalid. I do not think this is how it works, but is there a way to achieve this?

Polymer.dart <= 0.16.x

import 'dart:html';
import 'package:polymer/polymer.dart';
import 'package:core_elements/core_input.dart';

@CustomTag('app-element')

class AppElement extends PolymerElement {
  AppElement.created() : super.created() {}

  void inputHandler(Event e) {
    var inp = ($['custom'] as CoreInput);
    // very simple check - you can check what you want of courxe
    if(inp.inputValue.length < 5) {
      // any text is treated as validation error
      inp.jsElement.callMethod('setCustomValidity', ["Give me more!"]);
    } else {
      // empty message text is interpreted as valid input
      inp.jsElement.callMethod('setCustomValidity', [""]);
    }
  }
}

To validate only when the input element loses focus remove validateImmediately from the HTML element and use the on-change event instead (not tested).

<paper-input id="custom" on-input="{{inputHandler}}" validateImmediately></paper-input>

I added a comment at https://github.com/dart-lang/core-elements/pull/102 to make this method available directly in Dart with the next update.

The documentation of <core-input> states that the HTML5 constraint validation API is supported. For more information see https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/HTML5/Constraint_validation