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

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_asset_test.dart b/sdk/lib/_internal/pub/test/serve/missing_asset_test.dart
index 39efa2686cbc5dfceb66369822db46ddef6d8ebb..c173982d4ddce6731dec8829bf8a74fe3ca1fcaf 100644
--- a/sdk/lib/_internal/pub/test/serve/missing_asset_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/missing_asset_test.dart
@@ -15,7 +15,7 @@ main() {
       d.appPubspec()
     ]).create();
 
-    startPubServe();
+    pubServe();
     requestShould404("index.html");
     requestShould404("packages/myapp/nope.dart");
     requestShould404("assets/myapp/nope.png");