all: replace strings.Index with strings.Contains where possible

Change-Id: Ia613f1c37bfce800ece0533a5326fca91d99a66a
Reviewed-on: https://go-review.googlesource.com/18120
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
diff --git a/src/encoding/xml/marshal.go b/src/encoding/xml/marshal.go
index 8ebd693..9fcd5d7 100644
--- a/src/encoding/xml/marshal.go
+++ b/src/encoding/xml/marshal.go
@@ -850,7 +850,7 @@
 			switch k {
 			case reflect.String:
 				s := vf.String()
-				dashDash = strings.Index(s, "--") >= 0
+				dashDash = strings.Contains(s, "--")
 				dashLast = s[len(s)-1] == '-'
 				if !dashDash {
 					p.WriteString(s)