Please allow iron-icons as placeholders

[davidmaxwaterman]

I have an iron-iconset-svg and would like to use one of them as a placeholder for an iron-image, something like this:

  <iron-image placeholder="image:image" src="takes-ages.jpg"></iron-image>

https://jsbin.com/qomohexoxa/edit?html,output

I'm using iron-image#2.2.1.

I notice:

#84

showing how to use a data-uri, but I'd rather not duplicate the icons in my iron-iconset-svg.

Any way to do this that I'm missing?