<iron-signals>
iron-signals provides basic publish-subscribe functionality.
Note: avoid using iron-signals whenever you can use
a controller (parent element) to mediate communication
instead.
To send a signal, fire a custom event of type iron-signal, with
a detail object containing name and data fields.
this.fire('iron-signal', {name: 'hello', data: null});To receive a signal, listen for iron-signal-<name> event on a
iron-signals element.