format: esm breaks rollup static optimization

[ffflabs]

given a jspm.config.js with meta

 meta: {
    "*.css": {
      "defaultExtension": false,
      "format": "amd",
      "loader": "css"
    }
  }

Doing

jspm build style.css style.js

Works fine. And it works fine using format = "cjs" too. But if I try to use format = "esm" to take advantage of Rollup static optimization:

 meta: {
    "*.css": {
      "defaultExtension": false,
      "format": "esm",
      "loader": "css"
    }
  }

It errors with message:

Error loading styles/public.css: load hook should return a string, a { code, map } object, or nothing/null
/my_project/node_modules/rollup/dist/rollup.js:8839:10

This is actually the expected result if the build module is actually a function
https://github.com/rollup/rollup/blob/d1c1126b1aca6f5b44772b1ddc19b1651dc78928/test/function/load-returns-string-or-null/_config.js

But I guess when "format":"esm", the translated output should be delivered to Rollup in a special way

[guybedford]

This actually seems to work ok for me. This may have been fixed in the SystemJS builder patch recently, still pending release into jspm. The release should go out later today - if you can try again then, let me know if it seems fixed.