json: Marshal, Unmarshal using new scanner

R=r
CC=golang-dev
https://golang.org/cl/953041
diff --git a/src/pkg/expvar/expvar_test.go b/src/pkg/expvar/expvar_test.go
index 2dc399e..98cd9c2 100644
--- a/src/pkg/expvar/expvar_test.go
+++ b/src/pkg/expvar/expvar_test.go
@@ -61,7 +61,8 @@
 	// colours.String() should be '{"red":3, "blue":4}',
 	// though the order of red and blue could vary.
 	s := colours.String()
-	j, err := json.Decode(s)
+	var j interface{}
+	err := json.Unmarshal([]byte(s), &j)
 	if err != nil {
 		t.Errorf("colours.String() isn't valid JSON: %v", err)
 	}