Ran into this while analyzing the generated widget preview scaffold project, which failed because the pubspec contained a path with Windows path separators.

To reproduce on Windows, run:

// Note the use of POSIX path separators
dart pub add 'my_package:{"path":"C:/src/my_package"}'

The resulting entry in pubspec.yaml uses Windows path separators:

my_package:
  path: C:\src\my_package