Fix copying over client.dart.js to client.js when build DWDS (#2232)

Author: elliette

dwds/build.yaml

@@ -14,6 +14,7 @@ targets:
 
 builders:
   client_js_copy_builder:
+    required_inputs: [".js"]
     import: "tool/copy_builder.dart"
     builder_factories:
         - copyBuilder