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

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

Index: sdk/lib/_internal/pub/test/serve/unknown_dependency_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/unknown_dependency_test.dart b/sdk/lib/_internal/pub/test/serve/unknown_dependency_test.dart
index 2f66d144016b6c6bd3aa5889d4a8be9cec10d1bf..9f529584e5a56995063acebc1f941dbae6a28036 100644
--- a/sdk/lib/_internal/pub/test/serve/unknown_dependency_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/unknown_dependency_test.dart
@@ -15,7 +15,7 @@ main() {
       d.appPubspec()
     ]).create();
 
-    startPubServe();
+    pubServe();
     requestShould404("packages/foo/nope.dart");
     requestShould404("assets/foo/nope.png");
     requestShould404("dir/packages/foo/nope.dart");