Support parts and non-Dart files

[greglittlefield-wf]

When running in a large project, I hit some errors caused by

• trying to resolve the context for .DS_STORE
• trying to get the library declared in part files

These changes filter out non-Dart files and skip trying to get the library for part files.

After these changes, it worked like a charm!

Side note, this is a seriously awesome package, thanks for building it!!! ❤️

[glesica]

Thanks!