Unlike the other question that was posted where the Staging Environment had been bypassed in the Publishing Flow, the Dev environment cannot be bypassed.
However, here are a few things to check:
1. Make sure that the Dev Environment install code is the same as is configured on your website (Launch Property > Environment, Development > Install icon, check the Code against your environment..
2. Make sure your Publishing Flow actually has the Development environment selected... you can have multiple Dev environments, it's possible to save a library to any of your Dev environments, or to non-environment at all..
if those look correct, you could try removing the environment and saving your library, then selecting the Dev environment again and re-building...
I had a similar experience a year or so ago with a Demo site I didn't work on often... I don't recall what ended up working in the end, but after a few rebuilds and double checks, I was able to get it to properly build and not 404...
