ReadableStream.from for browsers is currently only supported in Firefox. I found the Node implementation in JavaScript, but it uses a lot of internal native node functions. Likewise, ReadableStream.from.toString() in Firefox's console reveals that its implementation is also a native function.
Is there a pure JavaScript implementation I can polyfill in temporarily? Perhaps using ReadableStreams constructor? I imagine the appropriate pull and cancel callbacks can be written based on the underlying iterator.
