Closure doesn't yet support package.json browser field: https://github.com/google/closure-compiler/issues/2433
However, our, module_deps.js / brower-resolve, and use this field.
One package that has problems is https://github.com/PaulLeCam/react-leaflet which depends on https://github.com/BerkeleyTrue/warning which uses browser entry.
index-node-modules-dir reuturns a list of all JS files in node_modules, this includes both warning/warning.js and warning/browser.js. index-node-modules only includes warning/browser.js. Because Closure doesn't understand package.json browser field, it can't resolve the warning require when processing React-leaflet: https://github.com/PaulLeCam/react-leaflet/blob/master/lib/Pane.js#L47