Url::fromUserInput() returning base scheme that cannot be resolved by FileSystem::realpath()

We're collecting an image path from the user (not unlike what the theme settings use for favicons + logos) as opposed to uploading a file, and have discovered that we are getting empty values trying to resolve this to a system-absolute path using the following mechanism:

~ # drush ev "echo \Drupal\Core\Url::fromUserInput('/themes/custom/league_amp/logo.gif')->getUri() . \"\n\";"
base:themes/custom/league_amp/logo.gif
~ # drush ev "echo \Drupal::service('file_system')->realpath('base:themes/custom/league_amp/logo.gif') . \"\n\";"

~ #

Digging a bit, it looks like the scheme validation is failing. I'm also unable to find a stream wrapper to handle the base scheme in core.

Anyone know if this is a known issue or if I'm just mixing incompatible calls? It really seems like the Url class should be returning a local URI that the FileSystem class can properly process...