More minor type improvements in dart:async.

sdk/lib/async/zone.dart

Index: sdk/lib/async/zone.dart
diff --git a/sdk/lib/async/zone.dart b/sdk/lib/async/zone.dart
index 48f156d42cb852c4e832f8fba6b881a7eeef4af9..7720be97140645f4a81d48a6f3f223efae3b6d1a 100644
--- a/sdk/lib/async/zone.dart
+++ b/sdk/lib/async/zone.dart
@@ -1437,7 +1437,7 @@ R runZoned<R>(R body(),
     errorHandler = (Zone self, ZoneDelegate parent, Zone zone, error,
         StackTrace stackTrace) {
       try {
-        if (onError is ZoneBinaryCallback<R, dynamic, StackTrace>) {
+        if (onError is ZoneBinaryCallback<R, Object, StackTrace>) {
           return self.parent.runBinary(onError, error, stackTrace);
         }
         return self.parent.runUnary(onError, error);