Fix fidlgen_dart logic

Before, the hasFidlgenDart flag was always false by mistake. I changed
it to just test fidlgenDart == "" instead of having a separate flag.

Change-Id: Ic4e0faee38e227d5ae7bf9787cb501e4fff891ed
Reviewed-on: https://fuchsia-review.googlesource.com/c/fidlbolt/+/990713
Reviewed-by: Clayton Wilkinson <wilkinsonclay@google.com>
diff --git a/backend/server.go b/backend/server.go
index beb3f7f..91311b7 100644
--- a/backend/server.go
+++ b/backend/server.go
@@ -25,7 +25,6 @@
 	fidlgenGo, clangFormat, rustfmt program
 	// Optional external programs.
 	fidlgenDart    program
-	hasFidlgenDart bool
 	// External files used by fidlbolt.
 	rustfmtToml string
 	// Cache of FIDL library information.
@@ -87,7 +86,6 @@
 	s.fidlgenDart, err = findProgram("fidlgen_dart", bin)
 	if err != nil {
 		log.Printf("warning: %s", err)
-		s.hasFidlgenDart = false
 	}
 	s.rustfmtToml, err = findFile("rustfmt.toml", etc)
 	if err != nil {
@@ -481,7 +479,7 @@
 	if !res.Ok {
 		return res, nil
 	}
-	if r.OutputMode == Dart && !s.hasFidlgenDart {
+	if r.OutputMode == Dart && s.fidlgenDart.path == "" {
 		res.Content = "This build of fidlbolt does not include fidlgen_dart"
 		res.Ok = false
 		return res, nil