Estou desenvolvendo um aplicativo no Kotlin e continuo recebendo o erro:

Falha ao resolver: firebase-auth-15.0.0`

ao tentar sincronizar o gradle. Estou tentando implementar um recurso de login do google.

apply plugin: 'com.android.application'

apply plugin: 'kotlin-android'

apply plugin: 'kotlin-android-extensions'
apply plugin: 'com.google.gms.google-services'

android {
compileSdkVersion 28
defaultConfig {
    applicationId "nus.is3261.kotlinapp"
    minSdkVersion 21
    targetSdkVersion 28
    versionCode 1
    versionName "1.0"
    testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
}
buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
}
}

dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
implementation 'com.android.support:appcompat-v7:28.0.0'
implementation 'com.android.support.constraint:constraint-layout:1.1.3'
implementation 'com.android.support:support-v4:28.0.0'
implementation 'com.android.support:design:28.0.0'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'com.android.support.test:runner:1.0.2'
androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
implementation 'com.google.firebase:firebase-auth:16.0.1:15.0.0'

}

Tentei seguir a solução no estouro de pilhaAqu, mas fiquei com esse aviso depois de sincronizar o gradleWarning: The app gradle file must have a dependency on com.google.firebase:firebase-core for Firebase services to work as intended.