According to the specs, Fetch should be able to handle ReadableStream as request body. Implementing this would likely result in some changes to ReadableStream and ReadableStreamController facades - ReadableStream needs a constructor that implements the underlyingSource, which allows interaction with ReadableStreamController, and ReadableStreamController should not have its own constructor, as it is constructed by ReadableStream during its construction.

If implementing this would be approved, I could give it a shot.

References:

• https://developer.mozilla.org/en-US/docs/Web/API/fetch#parameters
• https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream/ReadableStream
• https://developer.mozilla.org/en-US/docs/Web/API/ReadableStreamDefaultController