strings: use LastIndexByte in LastIndex

Change-Id: I1add1b92f5c2688a99133d90bf9789d770fd9f05
Reviewed-on: https://go-review.googlesource.com/9503Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/os/file_plan9.go

@@ -319,7 +319,7 @@ func hasPrefix(s, prefix string) bool {
 	return len(s) >= len(prefix) && s[0:len(prefix)] == prefix
 }
 
-// Variant of LastIndex from the strings package.
+// LastIndexByte from the strings package.
 func lastIndex(s string, sep byte) int {
 	for i := len(s) - 1; i >= 0; i-- {
 		if s[i] == sep {

src/strings/strings.go

@@ -185,14 +185,7 @@ func LastIndex(s, sep string) int {
 	case n == 0:
 		return len(s)
 	case n == 1:
-		// special case worth making fast
-		c := sep[0]
-		for i := len(s) - 1; i >= 0; i-- {
-			if s[i] == c {
-				return i
-			}
-		}
-		return -1
+		return LastIndexByte(s, sep[0])
 	case n == len(s):
 		if sep == s {
 			return 0