Required input fields should not be marked invalid (red) automatically.

If an input field is required then it will automatically be marked as invalid. This should not happen on a page load, but once a user interacts with it. See solution below.

``` JS
/**
* Check the validity state and update field accordingly.
*
* @public
*/
MaterialTextfield.prototype.checkValidity = function () {
if (this.input_.validity.valid) {
this.element_.classList.remove(this.CssClasses_.IS_INVALID);
} else {
if (this.element_.getElementsByTagName('input')[0].value.length > 0) {
this.element_.classList.add(this.CssClasses_.IS_INVALID);
}
}
};
```