iron-list doesn't allow the user to <TAB> through all contents.

[timothyarmstrong]

Description

When tabbing through the items in the iron-list, the first few items are focused, but then (after I think iron-list dynamically adds more children) tabbing will skip to the next DOM element after the iron-list. I see the same behavior when using a screenreader (I used VoiceOver on OSX Chrome).

Expected outcome

The user should be able to tab through all items in the iron-list.

Actual outcome

Only the first items are focusable; the later items are skipped.

Live Demo

Try the iron-list that the playlist panel uses on https://www.youtube.com/watch?v=fPgs3qq9CGs&list=PLhcBPsRKi9mUNNaWjSd_xNN9B1IVbJkBQ

Browsers Affected

• [x] Chrome

[keanulee]

We can't control the tab order because child elements are recycled (e.g. the first element child might follow the last element child), but we do provide a tabIndex property that your child template can bind to so we can control keyboard focus through up/down arrows.