Ajax plugin install silently fails if FTP credentials aren't set

[morganestes]

Installing a plugin via Ajax fails if FS_METHOD isn't defined/set, but the UI indicates success.

Steps to reproduce:

• Visit ​http://src.wordpress-develop.dev/wp-admin/plugin-install.php and click the "Install Now" button on a plugin.
• Observe the button change to spinning red arrows, then to green checkmark; text changes to "Installed!".
• Browser network panel shows following response from admin-ajax.php:

  {"success":true,"data":{"install":"plugin","slug":"theme-check"}}
  

Trying to activate the same plugin by clicking the "More Details" link and the "Install Now" button takes me to the Connection Information screen where I'm prompted to enter FTP credentials.

Adding define( 'FS_METHOD', 'direct' ); to wp-config.php allows the Ajax install to complete and the plugin to activate without further prompting.

[dd32]

This is being worked on in #29820