Pattern: Importing implementation file from another package
Issue: -
The libraries inside lib are publicly visible: other packages are free to
import them. But much of a package's code is internal implementation libraries
that should only be imported and used by the package itself. Those go inside a
subdirectory of lib called src. You can create subdirectories in there if
it helps you organize things.
You are free to import libraries that live in lib/src from within other Dart
code in the same package (like other libraries in lib, scripts in bin,
and tests) but you should never import from another package's lib/src
directory. Those files are not part of the package's public API, and they
might change in ways that could break your code.
Example of incorrect code:
// In 'road_runner'
import 'package:acme/lib/src/internals.dart;