Fatal error on all Angular pages if an extension is missing

This is really weird. Steps to reproduce:

      0. Create from scratch a new Drupal/CiviCRM with Drupal 7.x and CiviCRM 4.7.19.

1. Install any extension, for example CiviDiscount.
2. Visit these URLs and observe that the page loads correctly: /civicrm/a/#/mailing/1 ("New Mailing"), /civicrm/a/#/cxn ("Connections")
3. Delete the directory containing the extension files, e.g., /var/www/d7cividev/sites/default/files/civicrm/ext/org.civicrm.module.cividiscount/
4. Repeat step 2; the pages still load correctly.
5. Refresh the extensions cache, e.g., click the Refresh button on the page at /civicrm/admin/extensions?reset=1&action=browse; on that page you'll see that the extension is reported as "missing", and you'll get some popup error messages about the missing files, too.
6. Repeat step 2; the pages present a fatal error: CRM_Extension_Exception_MissingException: Unknown extension: org.civicrm.module.cividiscount in CRM_Extension_Container_Collection->getContainer() (line 150 of /var/www/d7cividev/sites/all/modules/civicrm/CRM/Extension/Container/Collection.php).
7. Just for fun, visit a known nonexistent angular url, e.g., /civicrm/a/#/nothingtoseehere and observe the same fatal error.

I tried examining the output of debug_backtrace() but felt a little lost. If you have tips for debugging this, please post them and I'll look further.