core-animated-pages should be imported by app-router.html

[liebrand]

When using core-animated-pages, things break unless you import the core-animated-pages. I kind of expected app-router.html to be importing it, as it's really it's dependency, not the clients.

[erikringsmuth]

The router can't load it since there's no standard location for dependencies. There's no guarantee that people are using Bower as the dependency manager. Even Polymer's examples use a folder called components rather than bower_components.