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

sdk/lib/_internal/pub/lib/src/command/barback.dart

Index: sdk/lib/_internal/pub/lib/src/command/barback.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/barback.dart b/sdk/lib/_internal/pub/lib/src/command/barback.dart
index 72a50ba67325d1447a5ba8e26a3c7198c1327fa5..b7df78bd640cb6d32c3754afd6c2a1eb3dd71611 100644
--- a/sdk/lib/_internal/pub/lib/src/command/barback.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/barback.dart
@@ -88,10 +88,10 @@ abstract class BarbackCommand extends PubCommand {
 
     sourceDirectories.addAll(commandOptions.rest);
 
-    // Prohibit "lib" and "asset".
+    // Prohibit "lib".
     var disallowed = sourceDirectories.where((dir) {
       var parts = path.split(path.normalize(dir));
-      return parts.isNotEmpty && ["lib", "asset"].contains(parts.first);
+      return parts.isNotEmpty && parts.first == "lib";
     });
 
     if (disallowed.isNotEmpty) {