The next spidermonkey upgrade will bring in https://bugzilla.mozilla.org/show_bug.cgi?id=1795914, which removes the JS implementation. We will need to port https://bugzilla.mozilla.org/show_bug.cgi?id=1730589 and subsequent changes from https://searchfox.org/mozilla-central/source/dom/streams/ReadableStream.cpp before we can upgrade again.