I tried enabling the theme on a site off of 8.x latest and got the above error in writer.theme. Looks like, according to this change record https://www.drupal.org/node/2457593, these functions have moved.

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dandaman created an issue. See original summary.

  • bryanbraun committed deebb39 on 8.x-2.x 
    Issue #2575581 by bryanbraun: Update APIs to use several new Drupal 8...
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bryanbraun CreditAttribution: bryanbraun as a volunteer commented
Status: Active » Closed (fixed)

Good find, and thanks for providing a link to the change record.

I put in a fix for this (and a couple other issues) and released the changes as 8.x-2.2-beta3.