Reverted to using `part of` and fixed unit test.

Was there a specific reason why `lib/src/common/*.dart` did not use any `part of` directives? So I added them back in cause dart_analyzer would report errors if they did not exist and only warnings if they did exist when imported to multiple libraries. Unit tests ran without failure, please double check. BTW, if you setup a drone.io account and add DartVectorMath then make me an admin I would gladly setup the automated testing and analyzer.