}
	b1.WriteString(strings.Repeat("x", 89))
	if w1 != 1 {
		t.Fatalf("write 1200 + 89 'x's: got %d writes, want 1", w1)
	}
	io.Copy(b1, onlyReader{strings.NewReader(strings.Repeat("x", 700))})
	if w1 != 1 {
		t.Fatalf("write 1200 + 789 'x's: got %d writes, want 1", w1)
	}
	io.Copy(b1, onlyReader{strings.NewReader(strings.Repeat("x", 600))})
	if w1 != 2 {