all: revert "all: prefer strings.LastIndexByte over strings.LastIndex"

This reverts https://golang.org/cl/66372.

Updates #22148

Change-Id: I3e94af3dfc11a2883bf28e1d5e1f32f98760b3ee
Reviewed-on: https://go-review.googlesource.com/68431
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/src/encoding/xml/marshal.go b/src/encoding/xml/marshal.go
index 773f0f2..42133a7 100644
--- a/src/encoding/xml/marshal.go
+++ b/src/encoding/xml/marshal.go
@@ -333,7 +333,7 @@
 	// Pick a name. We try to use the final element of the path
 	// but fall back to _.
 	prefix := strings.TrimRight(url, "/")
-	if i := strings.LastIndexByte(prefix, '/'); i >= 0 {
+	if i := strings.LastIndex(prefix, "/"); i >= 0 {
 		prefix = prefix[i+1:]
 	}
 	if prefix == "" || !isName([]byte(prefix)) || strings.Contains(prefix, ":") {