I am constantly getting this issue. Android Studio is not resolving 11.2.0 latest version of Firebase.
This is my root level gradle,
   /* Top-level build file where you can add configuration options common to all sub-projects/modules.*/
buildscript {
    repositories {
        jcenter{
            url 'http://jcenter.bintray.com'
        }
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.3.3'
        classpath 'com.google.gms:google-services:3.1.0'
        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}
allprojects {
    repositories {
        jcenter()
        maven {
            url "https://maven.google.com"
        }
    }
}
task clean(type: Delete) {
    delete rootProject.buildDir
}
And app gradle is,
    apply plugin: 'com.android.application'
android {
    compileSdkVersion 26
    buildToolsVersion "26.0.1"
    defaultConfig {
        applicationId "com.cognisun.fcmdemo"
        minSdkVersion 23
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}
dependencies {
    compile 'com.android.support:appcompat-v7:26.+'
    compile 'com.android.support.constraint:constraint-layout:1.0.2'
    compile 'com.android.support:design:26.+'
    compile 'com.google.android.gms:play-services:11.2.0'
    compile 'com.google.firebase:firebase-core:11.2.0'
}
apply plugin: 'com.google.gms.google-services'
I have downloaded every essential sdk files and also followed this link.
Even why I am doing wrong. Any suggestions guys