Günter Zöchbauer answer is correct but he hasn't provided it with examples.
So to use pub/package/lib without publishing on pub.dev :
1. Local - Save in some local folder
dependencies:
library_name:
path: /path/to/library_name
2. Hosted - Pushed on Github, Gitlab etc.
dependencies:
library_name:
git: https://github.com/username/library_name
Or to target specific branch
dependencies:
library_name:
git:
url: https://github.com/username/library_name.git
ref: dev #branch name
Or to target specific commit
dependencies:
library_name:
git:
url: https://github.com/username/library_name.git
ref: e234072340 #commit reference id
Where 'library_name' has to be the same as the 'name' declared in pubspec.yaml of that pub.