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

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

Index: sdk/lib/_internal/pub/test/serve/serve_from_dependency_asset_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/serve_from_dependency_asset_test.dart b/sdk/lib/_internal/pub/test/serve/serve_from_dependency_asset_test.dart
index 08c76e02575c2514900ae947a2acc32b6c1515ac..f026d8aacd9c94d456c15edf4c93524dfc6c2707 100644
--- a/sdk/lib/_internal/pub/test/serve/serve_from_dependency_asset_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/serve_from_dependency_asset_test.dart
@@ -31,9 +31,9 @@ main() {
     requestShouldSucceed("assets/foo/foo.txt", "foo");
     requestShouldSucceed("assets/foo/sub/bar.txt", "bar");
 
-    // "assets" can be in a subpath of the URL:
-    requestShouldSucceed("foo/assets/foo/foo.txt", "foo");
-    requestShouldSucceed("a/b/assets/foo/sub/bar.txt", "bar");
+    // "assets" cannot be in a subpath of the URL:
+    requestShould404("foo/assets/foo/foo.txt");
+    requestShould404("a/b/assets/foo/sub/bar.txt");
     endPubServe();
   });
 }