App crashes when enabling proguard

Repro steps:
Set minifyEnabled to true

Expected behavior:
App continues to work

Actual behavior:
App crashes on startup with error like:
com.google.firebase.database.d: No properties to serialize found on class com.google.firebase.codelab.friendlychat.a

com.google.firebase.codelab.friendlychat.a is the FriendlyMessage class.

Annotating that class w/ @Keep locally works. Something like https://github.com/firebase/quickstart-android/blob/master/database/app/proguard-rules.pro may also work.