Flutter error: 'Could not find method android() for arguments'

I've been stuck for a bit now, not being able to manage how to solve this error when adding Firebase to android app. No problem with iOS so far, but android has been quite a pain. Could anyone help me out please?

This is what my project-level build.gradle looks like:

buildscript {
    repositories {
        google()
        jcenter()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:3.5.3'
        classpath 'com.google.gms:google-services:4.3.10'
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

rootProject.buildDir = '../build'
subprojects {
    project.buildDir = "${rootProject.buildDir}/${project.name}"
}
subprojects {
    project.evaluationDependsOn(':app')
}


task clean(type: Delete) {
    delete rootProject.buildDir
}

And my app-level build.gradle looks like this:

def localProperties =  new Properties()
def localPropertiesFile = rootProject.file('local.properties')
if (localPropertiesFile.exists()) {
    localPropertiesFile.withReader('UTF-8') { reader ->
        localProperties.load(reader)
    }
}


def flutterRoot = localProperties.getProperty('flutter.sdk')
if (flutterRoot == null) {
    throw  new FileNotFoundException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
}

def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
if (flutterVersionCode == null) {
    flutterVersionCode = '1'
}

def flutterVersionName = localProperties.getProperty('flutter.versionName')
if (flutterVersionName == null) {
    flutterVersionName = '1.0'
}

apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
apply plugin: 'com.android.application'


android {
    compileSdkVersion 29

    lintOptions {
        disable 'InvalidPackage'
    }

    defaultConfig {
        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
        applicationId "com.roimiranda.flash_chat"
        minSdkVersion 16
        targetSdkVersion 29
        versionCode flutterVersionCode.toInteger()
        versionName flutterVersionName
        multiDexEnabled true
    }

    buildTypes {
        release {
            // TODO: Add your own signing config for the release build.
            // Signing with the debug keys for now, so `flutter run --release` works.
            signingConfig signingConfigs.debug
        }
    }
}

flutter {
    source '../..'
}


dependencies {
    implementation 'androidx.multidex:multidex:2.0.0'
    implementation platform('com.google.firebase:firebase-bom:29.0.1')
    implementation 'com.google.firebase:firebase-analytics'
}

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