expvar: add (*Int).Set

R=golang-dev, adg, rsc
CC=golang-dev
https://golang.org/cl/2336044
diff --git a/src/pkg/expvar/expvar_test.go b/src/pkg/expvar/expvar_test.go
index dc173b9..3dfc55a 100644
--- a/src/pkg/expvar/expvar_test.go
+++ b/src/pkg/expvar/expvar_test.go
@@ -27,6 +27,11 @@
 	if s := reqs.String(); s != "4" {
 		t.Errorf("reqs.String() = %q, want \"4\"", s)
 	}
+
+	reqs.Set(-2)
+	if reqs.i != -2 {
+		t.Errorf("reqs.i = %v, want -2", reqs.i)
+	}
 }
 
 func TestString(t *testing.T) {