Selected tabindex doesn't update first time

[greenyouse]

Description

I think there's an issue with updating the tabindex on the first selection. It could be because Polymer.IronMenuBehaviorImpl.focusedItem isn't set when the element is initialized.

Polymer.IronMenuBehaviorImpl._focusedItemChanged is what would usually handle the tabindex behavior but it doesn't change the selected item's tabindex because the item is never set as focused.

Expected outcome

The tabindex of a selected tab starts as 0 but becomes -1 when another tab gets selected.

Actual outcome

The tabindex stays as 0 after selecting another tab for the first time.

Live Demo

http://jsbin.com/jagoruqabi/1/edit?html,output

Steps to reproduce

1. Render paper-tabs with a selected attribute.
2. Tap an adjacent tab.
3. Tap the selected tab.
4. Tap an adjacent tab.