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

tests/language/async_return_types_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "dart:async";
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
 Future foo1() async {
   return 3;
 }
 
 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";
 }
 
 // Future<int, String> is treated like Future<dynamic>
 Future<int, String> /// tooManyTypeParameters: static type warning
 foo4() async {
   return "String";
 }
 
 int /// wrongReturnType: static type warning, dynamic type error
 foo5() async {
   return 3;
 }
 
 Future<int> foo6() async { 
   // This is fine, the future is flattened
   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);
 }
 
 
 Iterable<int> foo8() sync* {
   yield 1;
   // Can only have valueless return in sync* functions.
   return
       8 /// return_value_sync_star: compile-time error
@@ skipping 16 matching lines @@
   Expect.equals(3, await foo5());
   Expect.equals(3, await await foo6());
   Expect.equals(3, await await foo7());
   Expect.listEquals([1], foo8().toList());
   Expect.listEquals([1], await foo9().toList());
 }
 
 main() {
   asyncTest(test);
 }