Dart Debug Extension builder fix (#2117)

Author: elliette

dwds/debug_extension_mv3/build.yaml

@@ -13,6 +13,7 @@ targets:
 
 builders:
   client_js_copy_builder:
+    required_inputs: [".js", ".png", ".html", ".css", ".json"]
     import: "tool/copy_builder.dart"
     builder_factories:
         - copyBuilder