[Polymer Pass] dynamic property names in `listeners` object

Feature request from a user in geo. They'd like to be able to define event names as constants, then reference them in `listeners` using ES6's dynamic property name syntax.

e.g.

```javascript
const eventName = 'event-name';

Polymer({
is: 'foo-bar',
listeners: {
[eventName]: 'handler'
},
handler() {
console.log('got an event-name');
}
});
```