js.js

jQuery( window ).off( 'beforeunload', wp.updates.beforeunload );

jQuery( window ).on( 'beforeunload', function(){
    return 'always prompt'
});
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yo.php

<?php
/*
Plugin Name: Replace wp-plugin beforeunload 
Plugin URI: http://wordpress.org/plugins/hello-dolly/
Description: This is not just a plugin, It is a pipe 
Author: Aaron Jorbin 
Version: 0.1
Author URI: http://aaron.jorb.in
*/

function jorbin_beforeunload_add_admin_scripts( $hook ) {

    if ( $hook == 'plugins.php' || $hook == 'plugins-install.php' ) {
		wp_enqueue_script(  'myscript', plugins_url( 'js.js', __FILE__ ), array( 'updates' ) );
    }
}
add_action( 'admin_enqueue_scripts', 'jorbin_beforeunload_add_admin_scripts', 10, 1 );