Unable to find property ‘application’ on class: IndexYamlReader

A quick note for today: some developers are experiencing the below error while attempting to vacuum an application’s indexes:

Error Details:
Line 0, column 10: Unable to find property 'application' on class:
com.google.appengine.tools.admin.AdminException: Unable to perform vacuum_indexes
    at com.google.appengine.tools.admin.AppAdminImpl.vacuumIndexes(AppAdminImpl.java:346)
    at com.google.appengine.tools.admin.AppCfg$VacuumIndexesAction.execute(AppCfg.java:1605)
    at com.google.appengine.tools.admin.AppCfg.executeAction(AppCfg.java:327)
    at com.google.appengine.tools.admin.AppCfg.<init>(AppCfg.java:210)
    at com.google.appengine.tools.admin.AppCfg.<init>(AppCfg.java:121)
    at com.google.appengine.tools.admin.AppCfg.main(AppCfg.java:117)

Google seems to have changed the index backend during the 1.8.6 App Engine release, and this is causing older SDK versions to break when they attempt to issue the vacuum indexes command. To fix this, update your SDK version to the latest available.