title: What's the reasoning behind "part" and "part of" in Dart libraries?
body: <p>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 state the library of which they are part.</p>
  
  <p>This bidirection seems unnecessary to me and I would expect that a reference from the library to the parts would be enough. Also, when adding or removing files from libraries, there are 2 places where modifications need to be made.</p>
  
  <p>Can anyone explain this?</p>