FieldPluginBase::renderAsLink() loses language prefix for tokenized paths

Problem/Motivation

• Add a field to a View where you can type in a path and render it as a link, and then type in a path that includes token (e.g., {{nid}}). Or, import the views.view.test_dropbutton.yml View, which has that (the "Custom text" field).
• Enable the Language module, add a language to the site, and then go to that View's page with a language prefix. E.g., if using the test_dropbutton View, go to http://example.com/en/test-dropbutton.
• Notice that the link (e.g., the "Custom text" link) takes you to a URL that no longer has that language prefix.
• The problem is that FieldPluginBase::renderAsLink() calls Url::fromUri('user-path:...') on the tokenized path, and while tokenized, that doesn't match a route, so falls back to 'base:', which does not apply language prefixing (since you don't want language prefixing for unrouted paths). Then tokens are replaced, but we're already in the base: scheme, so too late.

Proposed resolution

Run the path (technically a URI at the point in the code) through the path validator to convert to a routable Url() object.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Beta phase evaluation