Implement --file-names-only (#8788)

* flatc builds and seems to work, some of the extra targets are having linker errors

* fix build system

* pipeline failures

* un-rename files

* refactor to use unique_ptr

* typo

* rm make_unique, add comments

* fix cmake

---------

Co-authored-by: Wouter van Oortmerssen <aardappel@gmail.com>

src/idl_gen_dart.cpp

@@ -163,7 +163,8 @@ class DartGenerator : public BaseGenerator {
       code += "\n";
       code += kv->second;
 
-      if (!SaveFile(Filename(kv->first).c_str(), code, false)) {
+      if (!parser_.opts.file_saver->SaveFile(Filename(kv->first).c_str(), code,
+                                             false)) {
         return false;
       }
     }