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

pkg/analyzer/test/generated/strong_mode_test.dart

 // Copyright (c) 2016, 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.
 
 library analyzer.test.generated.strong_mode_test;
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
@@ skipping 1884 matching lines @@
   test_genericMethod_then() async {
     String code = r'''
 import 'dart:async';
 String toString(int x) => x.toString();
 main() {
   Future<int> bar = null;
   var foo = bar.then(toString);
 }
 ''';
     await resolveTestUnit(code);
+
     expectInitializerType('foo', 'Future<String>', isNull);
   }
 
   test_genericMethod_then_prefixed() async {
     String code = r'''
 import 'dart:async' as async;
 String toString(int x) => x.toString();
 main() {
   async.Future<int> bar = null;
   var foo = bar.then(toString);
@@ skipping 497 matching lines @@
 main() {
   var v = x;
   v; // marker
 }
 int x = 3;
 ''';
     await assertPropagatedAssignedType(code, typeProvider.intType, null);
     await assertTypeOfMarkedExpression(code, typeProvider.intType, null);
   }
 }