Report wrong async function result type as type error in checked mode (Fixes #26133).

tests/language/async_return_types_test.dart

Index: tests/language/async_return_types_test.dart
diff --git a/tests/language/async_return_types_test.dart b/tests/language/async_return_types_test.dart
index 1134e25b0038c1d9caf25cac9229d5972427410d..f8d194da0ce7649a0a34d764de2f681a7faa00b3 100644
--- a/tests/language/async_return_types_test.dart
+++ b/tests/language/async_return_types_test.dart
@@ -14,7 +14,7 @@ Future<int> foo2() async {
   return 3;
 }
 
-Future<int> /// wrongTypeParameter: static type warning
+Future<int> /// wrongTypeParameter: static type warning, dynamic type error
 foo3() async {
   return "String";
 }
@@ -35,7 +35,7 @@ Future<int> foo6() async {
   return new Future<int>.value(3);
 }
 
-Future<Future<int>> /// nestedFuture: static type warning
+Future<Future<int>> /// nestedFuture: static type warning, dynamic type error
 foo7() async {
   return new Future<int>.value(3);
 }