code: function readerToStream(reader) {
      return new ReadableStream({
          pull(controller) {
              return reader.read()
                  .then(res => {
                      if (res.done) {
                          controller.close();
                      } else {
                          controller.enqueue(res.value);
                      }
                  });
          },
          cancel(reason) { return reader.cancel(reason); }
      });
  }
docs: Adapt a Reader to an UnderlyingSource, for wrapping into a ReadableStream.
  
  @param {Reader} reader
  @return {ReadableStream}