firefly-iii/app/Providers/AppServiceProvider.php

<?php namespace FireflyIII\Providers;

use Illuminate\Support\ServiceProvider;

/**
 * Class AppServiceProvider
 *
 * @package FireflyIII\Providers
 */
class AppServiceProvider extends ServiceProvider
{

    /**
     * Bootstrap any application services.
     *
     * @return void
     */
    public function boot()
    {
        //
    }

    /**
     * Register any application services.
     *
     * This service provider is a great spot to register your various container
     * bindings with the application. As you can see, we are registering our
     * "Registrar" implementation here. You can add your own bindings too!
     *
     * @return void
     */
    public function register()
    {
        $this->app->bind(
            'Illuminate\Contracts\Auth\Registrar',
            'FireflyIII\Services\Registrar'
        );
    }

}