laravelでCall to undefined method Illuminate\Routing\RouteFileRegistrar::get()

bad

Route::prefix('admin')->namespace('Admin')->name('admin.')->group(function () {
    // Authentication Routes...
    $this->get('login', 'Auth\LoginController@showLoginForm')->name('login');
});

good

Route::prefix('admin')->namespace('Admin')->name('admin.')->group(function () {
    // Authentication Routes...
    Route::get('login', 'Auth\LoginController@showLoginForm')->name('login');
});