To be able to make visible your local asset files (images, javascript, css, etc.) to Flutter, you need to add them in the assets section of the pubspec.yaml file, otherwise they cannot be found.
Read the official Flutter documentation Adding assets and images for more details.

Example of a pubspec.yaml file:

...

# The following section is specific to Flutter.
flutter:

  # The following line ensures that the Material Icons font is
  # included with your application, so that you can use the icons in
  # the material Icons class.
  uses-material-design: true

  assets:
    - assets/index.html
    - assets/css/
    - assets/images/

...