What is a "weak route" in Symfony 2?
Worked it out. The Symfony2 plugin parses the appDevDebugProjectContainer.php file to get route and service information. However this file is only recompiled when the web page is loaded which means newly created routes are not included.
To get around this the plugin also parses the config files but this is not 100% reliable so routes discovered in this way are marked as "weak". Reloading the web page will make the warning go away.
I finally fixed this issue for my situation (Symfony 2.8 / PhpStorm 2016.3).
It's similar to @Atmarama 's solution but I think my version of PhpStorm (2016.3) is different and/or Symfony 2.8.
My solution also doesn't involve the deprecated setting. I hope this will help others:
In PhpStorm go to Languages & Frameworks > PHP > Symfony > Routing and add the path to appDevProjectContainerUrlGenerator.php.
In my case this is:
var/cache/dev/appDevDebugProjectContainerUrlGenerator.php
