Responding to urging from Heroku, the Rails maintainers have merged sanity checks from the sprockets_better_errors gem into Rails 4.1. See https://github.com/rails/sprockets-rails/pull/84
The intent is to reveal asset pipeline errors that you would see in production when you run the app in development mode.
You probably are using a javascript_include_tag in an application layout instead of placing your JavaScript files in the app/assets/javascripts/ folder.
You could move your files to the app/assets/javascripts/ folder.
Alternatively, you’ll need to update your config/application.rb file to include:
config.assets.precompile += %w( my_js )
Note that the filename should not include the .js file extension.