Polymer.IronA11yKeysBehavior provides a normalized interface for processing
keyboard commands that pertain to WAI-ARIA best practices.
The element takes care of browser differences with respect to Keyboard events
and uses an expressive syntax to filter key presses.
keyBindings prototype property to express what combination of keys
will trigger the event to fire.
key-event-target attribute to set up event handlers on a specific
keys-pressed event will fire when one of the key combinations set with the
keys property is pressed.