While trying to cross-compile some zio-json code for supporting streaming json-lines, I ran into the issue that java.io.BufferedWriter was not implemented in Scala-js.
I was also cross-compiling to Scala-native and there the BufferedWriter class is implemented. I tried to use the same implementation for Scala-js and it turns out it just works out of the box. This is the implementation: https://github.com/scala-native/scala-native/blob/d5d65e755a4fb40a1c562d50b046384000232b23/javalib/src/main/scala/java/io/BufferedWriter.scala#L4

Can this be included in Scala-js?
I could make a PR with this single file and include e.g. this test: https://github.com/scala-native/scala-native/blob/d5d65e755a4fb40a1c562d50b046384000232b23/unit-tests/shared/src/test/scala/org/scalanative/testsuite/javalib/io/BufferedWriterTest.scala#L10
Anything else?