Use File instead of File.fromUri

[Gaurav192]

Fixes #19
File.fromUri throws UnsupportedError if file doesn't exist

[shigomany]

If PR is not accepted for a long time, then you can add it to your dev_dependency:

dev_dependencies:
  
  icon_font_generator:
    git:
      url: https://github.com/Gaurav192/icon_font_generator.git
      ref: file_path_fix

And run in this way:

dart run icon_font_generator ...

[838]

This has fixed our issue. Hope get merged soon :)