What's the reasoning behind “part” and “part of” in Dart libraries?

Question

I was wondering what the reasoning is behind the way libraries and their content are defined. More specifically, a library needs to list all parts and the parts need to stat

Answer 1

You can find some informations in this thread.

From Gilad Bracha :

Originally, parts were not tied to their libraries. People complained because it was difficult to give good tool support (i.e., what scope are you in when you open a part but not a library that uses it?).