Found some more code referring to the special "asset" directory.

sdk/lib/_internal/pub/test/serve/missing_asset_test.dart

Index: sdk/lib/_internal/pub/test/serve/missing_asset_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/missing_asset_test.dart b/sdk/lib/_internal/pub/test/serve/missing_asset_test.dart
index c173982d4ddce6731dec8829bf8a74fe3ca1fcaf..3f39054c5536e8573e9fa280aedb468368d4e939 100644
--- a/sdk/lib/_internal/pub/test/serve/missing_asset_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/missing_asset_test.dart
@@ -18,9 +18,7 @@ main() {
     pubServe();
     requestShould404("index.html");
     requestShould404("packages/myapp/nope.dart");
-    requestShould404("assets/myapp/nope.png");
     requestShould404("dir/packages/myapp/nope.dart");
-    requestShould404("dir/assets/myapp/nope.png");
     endPubServe();
   });
 }