fix(icon): prevent parsing the same icon set multiple times (#9635)

This fixes an issue where requests for icons in a particular set made
while that set is being fetched result in the set being parsed into an
SVGElement multiple times. It occured because, while the same pending http
request was used for each icon, each icon added their own `map` onto
that request, will every `map` resulting in a (potentially expensive) parse. This change sets the cached icon earlier and checks it later.

Author: jelbourn