Fix the Dart language version for Fuchsia's build of the args package (flutter#52083)

This is required for the next Dart roll (see
flutter/engine#52077)

Author: jason-simmons

shell/platform/fuchsia/dart/BUILD.gn

@@ -124,7 +124,7 @@ dart_library("args") {
   package_root = "$dart_src/third_party/pkg/args"
   package_name = "args"
 
-  pubspec = "$package_root/pubspec.yaml"
+  language_version = "2.17"
 
   deps = []