Class Validation in Dart Polymers

My goal is to validate within the class definition and pass the errors to the UI.

Input Forms and Classes

My input form in HTML passes a String.  My class expects a DateTime or int.  I handle this by creating a public string variable and an internal variable (_variable) of the correct class. In the constructor, I parse the public string variable into the desired class.
I may try holding on to the public string in "quarantine" until the user submits the form and then set the internal variables.  I am not sure if this is good practice.

Validation

First I need to be able to call the validation method from my polymer element.  Dart.Polymer doesn't let me call the method directly with:
<on-event = "{{instance.method}}">

So I create a method in my polymer element.
bool FormDateValidation ()=> instance.method();

Parse throws a FormatException.  I want to catch and display the exception.  The method becomes:

bool FormDateValidation (){
try{
return instance.method();
} catch(e){
formError = e;}

Questions:

• Can HTML pass DateTime and int directly?
• How to throw and catch an exception if the String Variable can not be parsed to the desired class?