Uncaught TypeError: Cannot set property '_isDivider' of undefined

[tamsler]

When using core-list, I am seeing the following error in the JS console:

Uncaught TypeError: Cannot set property '_isDivider' of undefined

version: Polymer/core-list#0.5.5

[denevell]

Is this in a auto-binding template?

If so, or regardless, try putting in a {{index}} in core-list's template.

Fixed it for me.

Related to #78