Steps to reproduce (on mw.org, or any other wiki with the most recent version):
- Click the icon for the link dialog
- Enter some random nonsense into the the field for the page title
- Enter an invalid title, such as <
In step 2 the page should be flagged as missing, in step 3 as invalid.
Probably both a regression caused by linting all the js files. (https://gerrit.wikimedia.org/r/#/c/14014/)