I've filed a bug https://bugs.webkit.org/show_bug.cgi?id=96211
Not all the required files were zipped into devtools_frontend.zip probably because we had switched to lazy loading schema. I'll try to fix it tomorrow.
Regards,Please refer to my how to steps as described here: https://groups.google.com/forum/m/?fromgroups#!topic/google-chrome-developer-tools/L1nemsmtnJI
We will deprecate debug-devtools way once we publish a howto for the new way.
Regards
Pavel