use append

R=gri, r, r2
CC=golang-dev
https://golang.org/cl/2743042
diff --git a/src/pkg/flag/flag_test.go b/src/pkg/flag/flag_test.go
index 83bf7ee..5fb7649 100644
--- a/src/pkg/flag/flag_test.go
+++ b/src/pkg/flag/flag_test.go
@@ -161,10 +161,7 @@
 }
 
 func (f *flagVar) Set(value string) bool {
-	n := make(flagVar, len(*f)+1)
-	copy(n, *f)
-	*f = n
-	(*f)[len(*f)-1] = value
+	*f = append(*f, value)
 	return true
 }