tainer styling when the input value changes.

### Add-ons

Add-ons are child elements of a `<paper-input-container>` with the `add-on` attribute and
implements the `Polymer.PaperInputAddonBehavior` behavior. They are notified when the input value
or validity changes, and may implement functionality such as error messages or character counters.
They appear at the bottom of the input.

### Prefixes and suffixes
These are child elements of a `<paper-input-container>` with the `prefix`
or `suffix` attribute, an