Implement info.ServiceAccount() on devserver via info.AccessToken(...).

impl/prod/info.go

Index: impl/prod/info.go
diff --git a/impl/prod/info.go b/impl/prod/info.go
index a93e413af7fd401b563dc4917963ff942a1303c0..691f08186e6ab75d70a263a03c09c1ea337c4a83 100644
--- a/impl/prod/info.go
+++ b/impl/prod/info.go
@@ -99,6 +99,11 @@ func (g giImpl) ServerSoftware() string {
 	return appengine.ServerSoftware()
 }
 func (g giImpl) ServiceAccount() (string, error) {
+	if appengine.IsDevAppServer() {
+		// On devserver ServiceAccount returns empty string, but AccessToken works.
+		// We use it to grab developer's email.
+		return developerAccount(g.aeCtx)
+	}
 	return appengine.ServiceAccount(g.aeCtx)
 }
 func (g giImpl) SignBytes(bytes []byte) (keyName string, signature []byte, err error) {