Potential bug in v. 0.5.0

[domderen]

Hi,

I just downloaded the new version 0.5.0, and it looks that your have an error here:
https://github.com/Polymer/core-animated-pages/blob/master/core-animated-pages.html#L425

This line crushes with error:
Uncaught TypeError: undefined is not a function

I assume this is because of the same issue, as described here: http://stackoverflow.com/questions/26122043/paper-dropdown-menu-error-in-observe-js