Use the polling watcher by default in pub tests for now.

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

Index: sdk/lib/_internal/pub/test/serve/invalid_urls_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/invalid_urls_test.dart b/sdk/lib/_internal/pub/test/serve/invalid_urls_test.dart
index 4939fd6314703bf7dca9e7e669c4e72756e35ec8..be482454c73b531be5f3e9f359d9a1cf7bcbd35e 100644
--- a/sdk/lib/_internal/pub/test/serve/invalid_urls_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/invalid_urls_test.dart
@@ -36,7 +36,7 @@ main() {
     ]).create();
 
     pubGet();
-    startPubServe();
+    pubServe();
     requestShould404("packages");
     requestShould404("assets");
     requestShould404("packages/");