Fix regression in appengine.go (#949)

AUTHORS

@@ -27,6 +27,7 @@ Daniël van Eeden <git at myname.nl>
 Dave Protasowski <dprotaso at gmail.com>
 DisposaBoy <disposaboy at dby.me>
 Egor Smolyakov <egorsmkv at gmail.com>
+Erwan Martin <hello at erwan.io>
 Evan Shaw <evan at vendhq.com>
 Frederick Mayle <frederickmayle at gmail.com>
 Gustavo Kristic <gkristic at gmail.com>

appengine.go

@@ -12,12 +12,13 @@ package mysql
 
 import (
 	"context"
+	"net"
 
 	"google.golang.org/appengine/cloudsql"
 )
 
 func init() {
-	RegisterDialContext("cloudsql", func(_ context.Context, instance addr) (net.Conn, error) {
+	RegisterDialContext("cloudsql", func(_ context.Context, instance string) (net.Conn, error) {
 		// XXX: the cloudsql driver still does not export a Context-aware dialer.
 		return cloudsql.Dial(instance)
 	})