##Polymer.IronA11yKeysBehavior
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.
Use the keyBindings prototype property to express what combination of keys
will trigger the event to fire.
Use the key-event-target attribute to set up event handlers on a specific
node.
The keys-pressed event will fire when one of the key combinations set with the
keys property is pressed.