Fix widget.dart to work with the latest web_ui library. Specifically, query selectors need to use "…

test/performance/perf.dart

Index: test/performance/perf.dart
diff --git a/test/performance/perf.dart b/test/performance/perf.dart
index eae7206edcb971a84b16eecaa1751a49e87e548a..81db7e0368e2e46bde3415f5dc52458c8dfc231f 100644
--- a/test/performance/perf.dart
+++ b/test/performance/perf.dart
@@ -67,7 +67,7 @@ Future<bool> showItem(Element item, {ShowHideEffect effect, int duration, Effect
 
   final oldActiveChild = _activeItem;
   if(oldActiveChild == item) {
-    return new Future<bool>.immediate(true);
+    return new Future<bool>.value(true);
   }
 
   [oldActiveChild, item].forEach((e) => e.classes.remove(_dirClassPrev));