Is `firebase-core` required for Firebase on Android?
UPDATE: AUGUST 2019
The docs now show whether a specific product SDK requires firebase-analytics, if firebase-analytics is recommended, or if it is not needed at all.
firebase-core is no longer listed as a required dependency in the Android getting started guide and was removed from the list of available libraries.
Now adding firebase-analytics to your app is optional in the getting started guide. Products that do require firebase-core, such as In-App Messaging, list the firebase-core dependency in their setup guides.
ORIGINAL ANSWER:
This is a recent change as of May 23, 2018. Per the release notes:
Your app gradle file now has to explicitly list
com.google.firebase:firebase-coreas a dependency for Firebase services to work as expected.
This requirement is reflected in the "Set up {X} for Android" guides (example) in the step "Install the Firebase SDK". That step links to this page, which includes the requirement to add firebase-core.
edit 9/26/18: A summary of features that rely on Google Analytics for Firebase (and therefore firebase-core) can be found here.