fix #28008, fix #28009 implement FutureOr<T>

sdk/lib/async/future_impl.dart

Index: sdk/lib/async/future_impl.dart
diff --git a/sdk/lib/async/future_impl.dart b/sdk/lib/async/future_impl.dart
index 6e0b55b4db35efc829b9550b5e901a75c721ba30..4e6ba423446fee6bf1eebe606b319ffbc72e26c8 100644
--- a/sdk/lib/async/future_impl.dart
+++ b/sdk/lib/async/future_impl.dart
@@ -231,11 +231,11 @@ class _Future<T> implements Future<T> {
   }
 
   Future<E> then<E>(
-      dynamic/*E|Future<E>*/ f(T value), { Function onError }) {
+      FutureOr<E> f(T value), { Function onError }) {
     Zone currentZone = Zone.current;
     ZoneUnaryCallback registered;
     if (!identical(currentZone, _ROOT_ZONE)) {
-      f = currentZone.registerUnaryCallback<dynamic, T>(f);
+      f = currentZone.registerUnaryCallback<FutureOr<E>, T>(f);
       if (onError != null) {
         onError = _registerErrorHandler<T>(onError, currentZone);
       }