s of May 5th chrome os canary builds of rialto have been failing:
https://cros-goldeneye.corp.google.com/console/listBuild?boards=veyron-rialto&milestone=52&chromeOsVersion=&chromeVersion=&startTimeFrom=&startTimeTo=#/

error message:

21:31:44: INFO: Updating status with message veyron_rialto-release: The BuildPackages [afdo_use] stage failed: Cannot find prebuilts for chromeos-base/chromeos-chrome on veyron_rialto in https://uberchromegw.corp.google.com/i/chromeos/builders/veyron_rialto-release/builds/0


Don, this release building ungrouping CL looks like the likely cause:
https://chromium-review.googlesource.com/#/c/342577/

...veyron_rialto-release did not appear in the new list of builders


(copied from b/28670183)