[1.0.3] Do not resolve hash-only urls used for routing

[nippur72]

I think the error #651 that was fixed in 0.5 is back in 1.0.

I have an my-element.html that defines:

<a href="#del"></a>

but in the browser it's converted to my-element.html#del.

[nippur72]

I wonder if it's the case of avoiding resolution for urls starting with / too, like we had in 0.5:
https://github.com/Polymer/polymer/blob/c7169640c567fb9bc709207ecbd9f747f8df48d5/src/lib/url.js#L99-L101