Open ‘View Page’ Button in Editor in new Tab

Firstly it is best to not edit your wordpress core files inside wp-admin but rather modify them with filters/actions and hooks. Check the wordpress codex to see all the filters you could use. Here is the code for a filter that you can put into your theme functions file (/wp-content/themes/{the-theme-you-are-using}/functions.php) to make the view post use target="_blank"

function my_get_sample_permalink_html($a){
    return preg_replace("/<span id='view-post-btn'><a/","<span id='view-post-btn'><a target="_blank"",$a);
}
add_filter('get_sample_permalink_html','my_get_sample_permalink_html');