Only allow “assets” to appear in the root of a URL.

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

Index: sdk/lib/_internal/pub/test/serve/serve_from_app_asset_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/serve_from_app_asset_test.dart b/sdk/lib/_internal/pub/test/serve/serve_from_app_asset_test.dart
index 63d324b16f1fd4e269b139e34416042856e6408a..c4913d5f2482504e7e89be1a118489c33faf68bb 100644
--- a/sdk/lib/_internal/pub/test/serve/serve_from_app_asset_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/serve_from_app_asset_test.dart
@@ -25,9 +25,9 @@ main() {
     requestShouldSucceed("assets/myapp/foo.txt", "foo");
     requestShouldSucceed("assets/myapp/sub/bar.txt", "bar");
 
-    // "assets" can be in a subpath of the URL:
-    requestShouldSucceed("foo/assets/myapp/foo.txt", "foo");
-    requestShouldSucceed("a/b/assets/myapp/sub/bar.txt", "bar");
+    // "assets" cannot be in a subpath of the URL:
+    requestShould404("foo/assets/myapp/foo.txt");
+    requestShould404("a/b/assets/myapp/sub/bar.txt");
     endPubServe();
   });
 }