prefer lowercase library file names

[devoncarew]

Generally, the Dart convention is to use lower_case file names for libraries. So, https://github.com/sergiandreplace/planets-flutter/blob/master/lib/Theme.dart would be theme.dart.

https://www.dartlang.org/guides/language/effective-dart/style#do-name-libraries-and-source-files-using-lowercase_with_underscores

YMMV of course -