Problem/Motivation
Now that Symfony sessions are in place and available from $request->getSession(), the form and batch subsystems should be updated to use that instead of accessing $_SESSION directly.
Note that FormBuilder::buildForm() is used in many BrowserTestBase test cases. Both forms and batches are also used in the installer.
This issue is was extracted from #2473875: Convert uses of $_SESSION to symfony session retrieved from the request to improve reviewability.
Proposed resolution
Remaining tasks
User interface changes
None
API changes
None