Exception with absolute URLs on Polymer 1.6.1

[Odaeus]

Description

A very recent change in Polymer due to Polymer/polymer#2448 causes the following exception to be thrown when using an absolute URL as the image src:

VM1527:1 Uncaught TypeError: Failed to construct 'URL': Invalid URL
    at TypeError (native)
    at eval (eval at evaluate (unknown source), <anonymous>:1:1)
    at iron-image._computeImgDivARIALabel (http://localhost:3001/assets/application-658e89886904e8ba3c8a9b98211302f287e1e8e6f5f8aa17ac61a231bfd0ac6f.html:19972:31)
    at iron-image._annotatedComputationEffect (http://localhost:3001/assets/application-658e89886904e8ba3c8a9b98211302f287e1e8e6f5f8aa17ac61a231bfd0ac6f.html:4494:24)

I think this is because new URL() expects a full URL with domain and protocol but now Polymer.ResolveUrl.resolveUrl() is returning the original absolute path without modification.

I think the solution is to change

iron-image/iron-image.html

Line 342 in e5872b8

var pathComponents = (new URL(this._resolveSrc(this.src))).pathname.split("/");

to supply the second parameter for URL() (this.ownerDocument.baseURI).

Steps to reproduce

1. Add an iron-image element with an absolute path as the src attribute and no alt value set.

[httx]

Also happens when using paper-card (which imports iron-image) without the heading attribute.