The latest version of plugin seed is available in the download area of the fling site vSphere HTML5 Web Client
Version 1.0.0 is now based on SDK 6.7, using the new Javascript APIs. It allows to build plugins for vSphere Client 6.5u2 and 6.7 but it is no longer compatible with the vSphere Flex Client.
It also uses Angular 6 and Angular-CLI 6. There are many other changes, please see the change log if you were already using an older version.