Flow Type definitions

[NShahri]

At the moment the only way to export flow types is to rename .js files to .js.flow. It means you need to publish your source.

• facebook/flow#1996
• facebook/flow#6504
• Time to deprecate gen-flow-files facebook/flow#5871
• what's the future of flow gen-flow-files? facebook/flow#5519
• No simple way to author and share flow packages facebook/flow#1996

I am thinking about one of these solutions:

• Generate flow type definition manually (the same as flow-typed)
  • Keep it up to date manually (the same as TypeScript Definitions). This make sense for small packages like react-workbox.
  • It does not make sense to use flow types in js, and have the same definition for exporting in another file
• Convert flow types to flow comments
  • https://flow.org/blog/2015/02/20/Flow-Comments/
  • Needs more to do
  • There is a babel plugin: https://babeljs.io/docs/en/babel-plugin-transform-flow-comments
  • It can be added to both build/esm and build/cjs