In preparation for some refactoring that I will be doing to
`ProjectFileInvalidator.findInvalidated`, make its code a bit
clearer:
* Indicate which arguments may be null.
* Don't bother calling `FileStat.statSync` on the `.packages` file
  for the initial load.  This makes the checks for the `.packages`
  file consistent with those for other files.
* Use `DateTime.isAfter()` instead of comparing microseconds
  ourselves.

While I was touching this file, I also removed some unnecessary
comparisons to `false`. (`_ManifestAssetBundle.wasBuiltOnce()` is the
only implementation I can find of `AssetBundle.wasBuiltOnce()`, and
it never returns `null`.)