Looks like you're editing a compiled phar archive. If you want to make changes you have decompile an unmodified phar then you can edit the source an recompile.
No, that doesn't make life easier at all, it's even harder as you have to zip your source code, go to the website, upload it, download the phar and then move it into the folder... It's only useful if the plugin doesn't load in PM and you want to get the source code
I never got DevTools considering that the Eclipse IDE i have can straight off import the pocketmine phar and export a phar file directly xd