add google-services.json to android/app
add " classpath 'com.google.gms:google-services:4.3.3' " to android/build.gradle (dependencies)
add " apply plugin: 'com.google.gms.google-services' " to android/app/build.gradle
change minApi to 21
in pubsec.yaml
firebase_core: "^0.5.0+1"
firebase_auth: "^0.18.1+2"
cloud_firestore: "^0.14.1+3"
google_sign_in: ^4.5.3
initialize app before runapp()