iron-selector 1.0.8 doesn't work with more-routing

[BrknRobot]

After updating iron-selector to 1.0.8, it no longer works with more-routing.

See: googlearchive/more-routing#77

[sgoryachkin]

I believe that it is more-routing problem,
but iron-selector.items is not changed de facto during "iron-items-changed" event fired because the event is fired before this._updateItems() in Polymer.IronSelectableBehavior._observeItems

[bicknellr]

Thanks for filing this, I'm going to close it as a dup of #89, even though this came first, since the other has more description.