Handle missing files more gracefully in pub serve.

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_file_test.dart b/sdk/lib/_internal/pub/test/serve/missing_asset_test.dart
similarity index 92%
copy from sdk/lib/_internal/pub/test/serve/missing_file_test.dart
copy to sdk/lib/_internal/pub/test/serve/missing_asset_test.dart
index cfd7d07770f0a897041b63585fb774ef63a6f2e8..1f69b6c4471db387328852e83ff03d6865ad829b 100644
--- a/sdk/lib/_internal/pub/test/serve/missing_file_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/missing_asset_test.dart
@@ -12,7 +12,7 @@ import 'utils.dart';
 
 main() {
   initConfig();
-  integration("responds with a 404 for missing files", () {
+  integration("responds with a 404 for missing assets", () {
     d.dir(appPath, [
       d.appPubspec()
     ]).create();