Event.cancelBubble cannot be used for stopping event propagation in Polymer #275

var e = document.createEvent('Event');
e.initEvent('foo', true, true);
var cb = e.cancelBubble;
e.cancelBubble = !e.cancelBubble;
if (cb !== e.cancelBubble) {
  console.log('writable');
} else {
  console.log('readonly');
}