From @collinjackson: If you try to build an app that consumes a plugin with gmscore dependencies, it fails until you add something like this to build.gradle of the app:

classpath 'com.google.gms:google-services:3.0.0'
apply plugin: 'com.google.gms.google-services'

It would be better if you could specify these dependencies in the pubspec.yaml file of the plugin and than have the build step inject this into the app.