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>

src/runtime/debug/stack_test.go

@@ -59,7 +59,7 @@ func TestStack(t *testing.T) {
 }
 
 func check(t *testing.T, line, has string) {
-	if strings.Index(line, has) < 0 {
+	if !strings.Contains(line, has) {
 		t.Errorf("expected %q in %q", has, line)
 	}
 }