Fix snapshotter deps file output (#2593)

- [x] Add the path not the url to the dependencies

sky/tools/sky_snapshot/loader.cc

@@ -161,13 +161,14 @@ std::string Loader::GetFilePathForFileURL(std::string url) {
 }
 
 std::string Loader::Fetch(const std::string& url) {
-  base::FilePath path(GetFilePathForURL(url));
+  std::string url_path = GetFilePathForURL(url);
+  base::FilePath path(url_path);
   std::string source;
   if (!base::ReadFileToString(path, &source)) {
     fprintf(stderr, "error: Unable to find Dart library '%s'.\n", url.c_str());
     exit(1);
   }
-  dependencies_.insert(url);
+  dependencies_.insert(url_path);
   return source;
 }