##Polymer.IronValidatableBehavior
Use Polymer.IronValidatableBehavior to implement an element that validates user input.
Use the related Polymer.IronValidatorBehavior to add custom validation logic to an iron-input.
By default, an <iron-form> element validates its fields when the user presses the submit button.
To validate a form imperatively, call the form's validate() method, which in turn will
call validate() on all its children. By using Polymer.IronValidatableBehavior, your
custom element will get a public validate(), which
will return the validity of the element, and a corresponding invalid attribute,
which can be used for styling.
To implement the custom validation logic of your element, you must override
the protected _getValidity() method of this behaviour, rather than validate().
See this
for an example.
Changing the invalid property, either manually or by calling validate() will update the
aria-invalid attribute.