n, err := w.Write([]byte("0123456789"))
	if n != 10 || err != nil {
		t.Fatalf("Write returned (%v, %v), want (10, nil)", n, err)
	}

	// Use ReadFrom to read in some data.
	n2, err := w.ReadFrom(strings.NewReader("abcdef"))
	if n2 != 6 || err != nil {
		t.Fatalf("ReadFrom returned (%v, %v), want (6, nil)", n2, err)
	}
}

type emptyThenNonEmptyReader struct {
	r io.Reader
	n int
}