[fidlgen][dart] Avoid name clash on enum member V

This is a quick point fix. Larger solutions are being worked on in:
https://fuchsia-review.googlesource.com/c/topaz/+/266374

Change-Id: Idf432c26c4e37ca39373fbc65f9b35874c7e7f53
diff --git a/bin/fidlgen_dart/backend/templates/enum.tmpl.go b/bin/fidlgen_dart/backend/templates/enum.tmpl.go
index 11b3145..dfe7c50 100644
--- a/bin/fidlgen_dart/backend/templates/enum.tmpl.go
+++ b/bin/fidlgen_dart/backend/templates/enum.tmpl.go
@@ -11,8 +11,8 @@
 ///{{ . -}}
 {{- end }}
 class {{ .Name }} extends $fidl.Enum {
-  factory {{ .Name }}(int v) {
-    switch (v) {
+  factory {{ .Name }}(int _v) {
+    switch (_v) {
 {{- range .Members }}
       case {{ .Value }}:
         return {{ .Name }};