The LoadAdminUIServlet class loads up, and serves back, "admin.html", meaning it cannot be used in its current state to serve up the new admin UI.
An update is needed to this class to make it serve back whatever html file was requested in the URL. There will, likely, only ever be two of them mentioned in web.xml, but it would be really useful for changes to web.xml not to require Java code changes also.
I'm hoping that someone with an up-and-running Java coding setup can make this pretty trivial tweak. Any volunteers?
SOLR-7858 Make Angular UI default