For Android

To Register your Flutter App to Firebase, you need to follow the following steps:

Sign into Firebase using your Google account.
Create a Firebase project by clicking on Add project in the Firebase console and then entering a Project Name(e.g. flutterstarter).
To add an Android app, click the icon to launch the setup workflow. Enter com.flutterstarter.app in Android package name field.
Click Download google-services.json to obtain your Firebase Android config file(google-services.json).
Move your config file into the android/app directory of your Flutter app.
To enable Firebase services in your Android app, add the google-services plugin to your Gradle files, as follows:
1. In your root-level (project-level) Gradle file(app/android/build.gradle):
buildscript {

repositories {
// Check that you have the following line (if not, add it):
google() // Google's Maven repository
}

// ...

dependencies {
// ...

// Add the following line:
classpath 'com.google.gms:google-services:4.3.4' // Google Services plugin
}
}

allprojects {
// ...

repositories {
// Check that you have following line (if not, add it):
google() // Google's Maven repository
// ...
}
}
1. In your module (app-level) Gradle file (usually app/android/app/build.gradle), apply the Google Services Gradle plugin.
// Add the following line:
apply plugin: 'com.google.gms.google-services' // Google Services plugin

android {
// ...
}

// ...
Add Flutter plugins in pubspec.yaml file.

dependencies:
  flutter:
    sdk: flutter
  //Add plugins here
  cloud_firestore: ^0.12.9
  firebase_core: ^0.4.0+8